Objective with crystal lenses

ABSTRACT

Objective, in particular a projection objective for a microlithography projection-exposure installation, with at least one fluoride crystal lens. A reduction in the detrimental influence of birefringence is achieved if this lens is a (100)-lens with a lens axis which is approximately perpendicular to the {100} crystallographic planes or to the crystallographic planes equivalent thereto of the fluoride crystal. In the case of objectives with at least two fluoride crystal lenses, it is favorable if the fluoride crystal lenses are arranged such that they are rotated with respect to one another. The lens axes of the fluoride crystal lenses may in this case point not only in the &lt;100&gt; crystallographic direction but also in the &lt;111&gt; crystallographic direction or in the &lt;110&gt; crystallographic direction. A further reduction in the detrimental influence of birefringence is achieved by the simultaneous use of groups with (100)-lenses rotated with respect to one another and groups with (111)-lenses or (110)-lenses rotated with respect to one another. A further reduction in the detrimental influence of birefringence is obtained by covering an optical element with a compensation coating.

The invention relates to an objective according to the precharacterizing clause of claim 1.

Projection objectives of this type are known from U.S. Pat. No. 6,201,634, which discloses the concept of ideally aligning the lens axes perpendicularly to the {111} crystallographic planes of the fluoride crystals during the manufacture of fluoride crystal lenses in order to minimize stress-induced birefringence. U.S. Pat. No. 6,201,634 is based on the assumption that fluoride crystals have no intrinsic birefringence.

However, it is known from the Internet publication “Preliminary Determination of an Intrinsic Birefringence in CaF₂” by John H. Burnett, Eric L. Shirley, and Zachary H. Levine, NIST Gaithersburg, Md. 20899 USA (posted on May 7, 2001), that calcium fluoride single crystals also exhibit birefringence that is not stress-induced, i.e., intrinsic birefringence. The measurements presented there show that a ray of light traveling in the <110> direction of a calcium fluoride crystal is subject to a birefringence that amounts to (6.5±0.4) nm/cm at a wavelength of γ=156.1 nm, to (3.6±0.2) nm/cm at a wavelength of γ=193.09 nm, and to (1.2±0.1) nm/cm at a wavelength of γ=253.65 nm. On the other hand, if the light propagation is oriented in the <100> direction or in the <111> direction of the crystal, no intrinsic birefringence occurs in calcium fluoride, as is also predicted by theory. Therefore, the intrinsic birefringence has a strong directional dependence and increases significantly as the wavelength becomes less.

The indices for the crystallographic directions will hereinafter be given between the symbols “<” and “>”, and the indices for the crystallographic planes will be given between the symbols “{” and “}”. The crystallographic direction always points in the direction of the surface normal of the corresponding crystallographic plane. For example, the crystallographic direction <100> points in the direction of the surface normal of the crystallographic plane {100}. Crystals with a cubic lattice structure, which include the fluoride crystals, have the principal crystallographic directions <110>, < 110>, < 110>, < 1 10>, <101>, <10 1>, < 101>, < 10 1>, <011>, <0 11>, <0 11>,<0 1 1>,<111>, < 1 1 1>, < 1 11>, < 11 1>, <1 1 1>, < 111>, <1 11>, <11 1>, <100>, <010>, <001>, < 100>, <0 10>, and <00 1>. Because of the symmetry properties of cubic crystals, the principal crystallographic directions <100>, <010>, <001>, < 100>, <0 1 0>, and <00 1 > are equivalent to one another, so that crystallographic directions which point in one of these principal crystallographic directions will hereinafter be given the prefix “(100)-”. Crystallographic planes that are perpendicular to one of these principal crystallographic directions will correspondingly be given the prefix “(100)-”. The principal crystallographic directions <110>, < 110>, < 110>, < 1 10>, <101>, <10 1>, < 101>, < 10 1>, <011>, <0 11>, <01 1>, and <0 1 1> are similarly equivalent to one another, so that crystallographic directions which point in one of these principal crystallographic directions will hereinafter be given the prefix “(110)-”. Crystallographic planes that are perpendicular to one of these principal crystallographic directions will correspondingly be given the prefix “(110)-”. The principal crystallographic directions <111>, < 1 1 1>, < 1 11>, < 11 1>, <1 1 1>, < 111>, <1 11>, <11 1> are similarly equivalent to one another, so that crystallographic directions which point in one of these principal crystallographic directions will hereinafter be given the prefix “(111)-”. Crystallographic planes that are perpendicular to one of these principal crystallographic directions will correspondingly be given the prefix “(111)-”. Any statements made hereinafter in regard to one of the aforementioned principal crystallographic directions should be understood to be equally applicable to the equivalent principal crystallographic directions.

Projection objectives and microlithography projection-exposure installations are known for example from the applicant's patent application PCT/EP00/13148 and from the references cited in it. The exemplary embodiments of that application show suitable projection objectives of purely refractive as well as catadioptric types with numerical apertures of 0.8 and 0.9 at operating wavelengths of 193 nm and 157 nm.

The concept of rotating lens elements in order to compensate for the effects of birefringence is also described in the patent application “Projektionsbelichtungsanlage der Mikrolithographie, Optisches System und Herstellverfahren” (Projection-exposure Installation for Microlithography, Optical System and Manufacturing Method) (DE 101 23 725.1), which has the applicant's file reference 01055P and was filed on May 15, 2001. The content of that application is hereby incorporated by reference in the present application.

The object of the invention is to provide projection objectives for a microlithography projection-exposure installation in which the influence of birefringence, in particular intrinsic birefringence, is significantly reduced.

This object is achieved by an objective according to claim 1, 8 and 31, a microlithography projection-exposure installation according to claim 47, a method for producing semiconductor components according to claim 48, a method for producing objectives according to claim 49, a method of compensating for birefringence effects according to claim 53 and a method of manufacturing lenses according to claim 54.

Advantageous refinements of the invention become evident from the features of the dependent claims.

To minimize the influence of intrinsic birefringence, claim 1 proposes aligning the lens axes in fluoride crystal lenses in the <100> crystallographic direction. The lens axes are considered to be aligned in a principal crystallographic direction if the lens axis deviates by no more than 5° from the principal crystallographic direction. It should be noted that not all the fluoride crystal lenses of the objective have to have such an alignment of the crystallographic planes. Those lenses in which the lens axes are perpendicular to the {100} crystallographic planes will hereinafter also be referred to as (100)-lenses. The alignment of the lens axis in the <100> crystallographic direction has the advantage that the detrimental influence of the intrinsic birefringence that affects light rays traveling in the <110> crystallographic direction becomes noticeable only at larger aperture angles of the light rays than would be the case with an alignment of the lens axis in the <111> crystallographic direction. Aperture angle is understood in this connection as meaning the angle between a light ray and the optical axis outside a lens and between the light ray and the lens axis inside a lens. The corresponding light rays are affected by birefringence only when the aperture angles enter the angular range between the <100> crystallographic direction and the <110> crystallographic direction. The angle between the <100> crystallographic direction and the <110> crystallographic direction is 45°. If on the other hand, the lens axis were aligned in the <111> crystallographic direction, the detrimental influence of intrinsic birefringence would become noticeable already at smaller aperture angles, because the angle between the <110> crystallographic direction and the <111> crystallographic direction is only 35°.

Of course, the inventive concepts disclosed herein are likewise applicable to lessen the detrimental influence of birefringence in cases where the angular dependency of the birefringence is caused for example by the manufacturing process of the fluoride crystal or the mechanical loading of the lens, in particular stress-induced birefringence.

The lens axis may be defined for example by a symmetry axis of a rotationally symmetrical lens. If the lens does not have a symmetry axis, the lens axis may be defined as the center of an incident bundle of rays, or by a straight line in relation to which the ray angles of all light rays within the lens are minimal. The lenses may be for example refractive or diffractive lenses as well as correction plates with free-form corrective surfaces. Planar-parallel plates are likewise considered as lenses if they are arranged in the path of rays of the objective. In a planar-parallel plate, the lens axis is perpendicular to the planar lens surfaces.

Preferably, however, the lenses are rotationally symmetrical lenses.

Objectives have an optical axis which runs from the object plane to the image plane. The (100)-lenses are preferably constructed such that they are centered about this optical axis, so that the lens axes also coincide with the optical axis.

An advantageous application of the invention is in projection objectives for a microlithography projection-exposure installation, because these objectives must meet extremely stringent requirements with respect to their optical resolution. But birefringence also has a detrimental influence on lens-testing objectives that are used for example to test lenses for projection objectives by measuring wave fronts of large aperture.

In objectives with large numerical apertures on the image side, particularly of more than 0.7, there will be aperture angles inside the (100)-lenses that exceed 25°, in particular exceed 30°. The inventive concept of orienting the lens axes in the <100> crystallographic direction is used to particular advantage with these large aperture angles. If the lens axes were oriented in the <111> crystallographic direction, the light rays with aperture angles exceeding 25°, in particular exceeding 30°, would be more noticeably affected by the detrimental influence of birefringence unless one of the corrective measures described farther below is used.

Since, on the other hand, the detrimental influence of intrinsic birefringence can reach a maximum at an aperture angle of 45°, and it is advantageous to design the projection objective in such a way that all the aperture angles of the light rays are smaller than 45°, in particular no larger than ${\arcsin\left( \frac{NA}{n_{FK}} \right)},$ where NA denotes the numerical aperture on the image side and n_(FK) denotes the refractive index of the fluoride crystal. The expression $\arcsin\left( \frac{NA}{n_{FK}} \right)$ indicates the aperture angle that corresponds to the image-side numerical aperture inside a fluoride crystal lens if the light ray is refracted at a planar boundary surface. This is achieved if the lenses that are arranged near the image plane have surfaces that converge the light rays, planar surfaces, or surfaces that diverge the passing light rays only slightly, provided that the light-diverging lens surface is followed in the direction of the light by a lens surface that converges the light rays more strongly.

Large aperture angles occur primarily with lenses that are arranged near field planes, in particular near the image plane. The (100)-lenses should therefore preferably be used in the region of the field planes. The region in which the (100)-lenses should be used can be determined on the basis of the ratio between the lens diameter and the diameter of the diaphragm. Therefore, the lens diameter of the (100)-lenses is preferably no more than 85%, in particular no more than 80%, of the diaphragm diameter.

As a rule, the largest aperture angles in projection objectives occur in the lens closest to the image plane. This is therefore the preferred lens in which to align the lens axis in the <100> crystallographic direction.

In addition to its dependence on the aperture angle of a light ray, the intrinsic birefringence of a fluoride crystal lens is also dependent on the azimuth angle of the light ray. Therefore, every fluoride crystal lens can be assigned a birefringence distribution Δn (α_(L), θ_(L)) which is on the one hand a function of the aperture angle θ_(L) and on the other hand a function of the azimuth angle α_(L). For a given ray direction that is determined by the aperture angle θ_(L) and the azimuth angle α_(L), the value of the birefringence Δn (expressed in [nm/cm]) indicates the optical path difference for two mutually orthogonal states of linear polarization in relation to the physical path length traveled by the light ray inside the fluoride crystal. The intrinsic birefringence is consequently independent of the path lengths and of the shape of the lens. The optical path difference for a ray is correspondingly obtained by multiplying the birefringence by the path length traveled. The aperture angle θ_(L) represents the angle that a light ray encloses with the lens axis, while the azimuth angle α_(L) represents the angle between the projection of the light ray into the crystallographic plane perpendicular to the lens axis and a reference direction inextricably linked with the lens.

Because the birefringence distributions of the individual fluoride crystal lenses are angle-dependent, the rays of a bundle that converges at an image point in the image plane of the objective have angle-dependent optical path differences ΔOPL (α_(R), θ_(R)) for two mutually orthogonal states of linear polarization. The optical path differences ΔOPL are in this case given as a function of the aperture angle θ_(R) and the azimuth angle α_(R). The aperture angle θ_(R) of a light ray is in this case defined as the angle between the direction of the ray and the optical axis in the image plane, and the azimuth angle α_(R) is defined as the angle between the projection of the light ray into the image plane and a fixed reference direction within the image plane. If the objective has at least two lenses or lens parts of fluoride crystal, it is advantageous if the lens axes of these lenses or lens parts point in a principal crystallographic direction and the lenses or lens parts are arranged such that they are rotated with respect to one another about the lens axes in such a way that the distribution ΔOPL (α_(R), θ_(R)) of the optical path differences has significantly reduced values in comparison with an arrangement in which the lens axes point in the same principal crystallographic direction and the lenses or lens parts are installed with uniform orientation. Since, however, the birefringence distributions of the lenses have an azimuthal dependence, the rotated arrangement of the lenses can reduce the maximum value of the distribution ΔOPL (α_(R), θ_(R)) by up to 20%, in particular by up to 25%, in comparison with a uniformly oriented installation.

Lens parts are understood for example as meaning individual lenses that are joined together in an optically seamless manner to form one individual lens by wringing. In the most general sense, lens parts refer to the components of an individual lens, the lens axes of the lens parts respectively pointing in the direction of the lens axis of the individual lens.

By installing the fluoride crystal lenses with mutually rotated orientations, the dependence of the distribution ΔOPL (α_(R), θ_(R)) on the azimuth angle α_(R) can be significantly reduced, so that an almost rotationally symmetrical distribution ΔOPL (α_(R), θ_(R)) is obtained.

If the lens axis is pointing in a principal crystallographic direction, the birefringence distribution Δn (α_(L), θ_(L)) of the lens will have a k-fold azimuthal symmetry. For example, the birefringence distribution of a (100)-lens, in which the lens axis points in the <100> crystallographic direction, has a 4-fold azimuthal symmetry, the birefringence distribution of a (111)-lens, in which the lens axis points in the <111> crystallographic direction, has a 3-fold azimuthal symmetry, and the birefringence distribution of a (110)-lens, in which the lens axis points in the <110> crystallographic direction, has a 2-fold azimuthal symmetry. Depending on the order of the azimuthal symmetry, the individual lenses or lens parts of a group are arranged with specific angles of rotation γ at which they are rotated with respect to one another about the lens axes. The angles of rotation γ are in this case measured between the reference directions of respective pairs of lenses or lens parts. The lens axes for the lenses of a group point in the same principal crystallographic direction or a principal crystallographic direction equivalent thereto. The reference directions of the lenses of a group are linked with the lenses in such a manner that the birefringence distributions Δn (α_(L), θ₀) for a given aperture angle θ₀ have the same azimuthal profile. Consequently, the azimuthal regions with maximum birefringence occur at the same azimuth angles for all of the lenses of a group. For n lenses of a group, the angles of rotation between respective pairs of lenses are given as follows: $\gamma = {\frac{360{^\circ}}{k\quad n} + {{m\quad\frac{360{^\circ}}{k}} \pm {10{^\circ}}}}$ where k stands for the degree of azimuthal symmetry, n for the number of lenses of a group and m for an arbitrary integral number. The tolerance of ±10° allows for the fact that the angles of rotation may deviate from the theoretically ideal angles, so that other constraints can be taken into account in the adjustment of the objective. A deviation from the ideal angle of rotation leads to non-optimized azimuthal compensation of the optical path differences of the lenses of a group. This can, however, be tolerated within certain limits. For (100)-lenses, the following specification is consequently obtained for the angles of rotation: $\gamma = {\frac{90{^\circ}}{n} + {{{m \cdot 90}{^\circ}} \pm {10{{^\circ}.}}}}$ If the group is made up of two (100)-lenses, the angle of rotation between these two lenses will ideally be 45° or 135°, 225°, . . . For (111)-lenses, the following specification is consequently obtained for the angles of rotation: $\gamma = {\frac{120{^\circ}}{n} + {{{m \cdot 120}{^\circ}} \pm {10{{^\circ}.}}}}$ For (110)-lenses, the following specification is consequently obtained for the angles of rotation: $\gamma = {\frac{180{^\circ}}{n} + {{{m \cdot 180}{^\circ}} \pm {10{{^\circ}.}}}}$ However, the distribution of the optical path differences ΔOPL_(G) (α_(R), θ_(R)) can also be stated for the influence of an individual group of lenses, by only these lenses being considered in the evaluation of the birefringence and the other lenses being assumed to have no birefringence.

The lenses of a group are determined for example by an outermost aperture ray of a bundle of rays within these lenses having in each case similar aperture angles, the aperture angles of the outermost aperture ray within these lenses advantageously being greater than 15°, in particular greater than 20°. The outermost aperture ray refers to a light ray that originates from an object point and whose height in the diaphragm plane corresponds to the radius of the diaphragm, and which consequently has in the image plane an angle according to the numerical aperture on the image side. The outermost aperture rays are used to define the groups because they normally have the largest aperture angles inside the lenses and are therefore most affected by birefringence. The determination of the optical path difference for two mutually orthogonal states of linear polarization for the outermost aperture rays consequently provides information about the maximum extent to which a wave front is affected by the birefringence.

It is further advantageous if the path lengths traveled by the outermost aperture ray are equal in each of these lenses. These measures provide good compensation of the azimuthal contributions to the distribution of the optical path differences that are caused by the individual lenses of a group, so that the resulting distribution of the optical path differences comes close to being rotationally symmetrical.

It is further of advantage if the outermost aperture ray has approximately equal optical path differences for two mutually orthogonal states of linear polarization in each lens of a group if the lenses are arranged with the same orientation. If this condition is met, the mutually rotated arrangement of these lenses will provide an optimized compensation of the azimuthal contributions.

In the case of planar-parallel adjacent (100)-or (111)-lenses of equal thickness or of four planar-parallel adjacent (110)-lenses of equal thickness, a rotationally symmetrical distribution of the optical path differences ΔOPL is obtained by rotating the lenses in accordance with the above formulas. In the case of lenses with curved surfaces, it is likewise possible to achieve an approximately rotationally symmetrical distribution of the optical path differences even by rotating only two lenses if the lenses of a group are carefully selected or the thicknesses and radii of the lenses are appropriately selected. In the case of (100)-lenses or (111)-lenses, it is advantageous if a group has two lenses. In the case of (110)-lenses, an approximately rotationally symmetrical distribution of the optical path differences is achieved for four lenses in a group.

The measure of rotating the lenses is particularly effective if the lenses are arranged adjacently. It is particularly advantageous to divide a lens into two parts and to join the lens parts together, such that they are rotated with respect to one another, in an optically seamless manner, for example by wringing.

In the case of a projection objective with a multitude of lenses, it is beneficial to form several groups of lenses, where the lenses of a group are arranged such that they are rotated about the lens axes in such a way that the resulting distribution ΔOPL (α_(R), θ_(R)) is virtually independent of the azimuth angle.

While the mutual rotation of the lenses of a group makes the distributions ΔOPL_(G) (α_(R), θ_(R)) caused by the individual groups virtually independent of the azimuth angle, the maximum value of the overall distribution ΔOPL (α_(R), θ_(R)) of the entire objective can be significantly reduced by the projection objective having both at least one group with (100)-lenses and at least one group with (111)-lenses. Good compensation is also possible if a group with (110)-lenses is arranged within the objective in addition to a group with (100)-lenses.

The reason why the compensation is possible is that the birefringence has not only an absolute value but also a direction. The detrimental influence of the birefringence is optimally compensated if the distribution of the optical path differences ΔOPL₁ (α_(R), θ_(R)) caused by the lenses or lens parts of all of the groups with (100)-lenses has maximum values of similar magnitude to those of the distribution of the optical path differences ΔOPL₂ (α_(R), θ_(R)) caused by the lenses or lens parts of all of the groups with (111)- or (110)-lenses.

A further advantageous possibility of reducing the detrimental influence of the birefringence is to cover an optical element of the projection objective with a compensation coating. This is based on the known fact that every optical coating, for example antireflection or mirror coatings, not only have properties with respect to reflection and transmission but also always produce optical path differences for two mutually orthogonal states of linear polarization. The effect is different depending on whether the light is s-polarized or p-polarized and depends furthermore on the angle of incidence at which the light ray meets the coating. Therefore, the birefringence in this case depends on the angle of incidence. For a bundle of rays whose central ray meets the compensation coating at an angle of incidence of 0°, the values and directions of the birefringence are rotationally symmetrical in relation to the central ray. The compensation coating is designed to produce a specific amount of birefringence as a function of the aperture angle of the light rays of a bundle of rays.

As a first step for this, the distribution of the optical path differences ΔOPL (α_(R), θ_(R)) is determined for two mutually orthogonal states of linear polarization for a bundle of rays in the image plane of the projection objective. The aperture angle θ_(R) of a light ray is thereby determined between the direction of the ray and the optical axis in the image plane, the azimuth .angle α_(R) between the projected direction of the ray into the image plane and a fixed reference direction within the image plane. In this case, the distribution of the optical path differences ΔOPL (α_(R), θ_(R)) for two mutually orthogonal states of linear polarization describes all the influences brought about by intrinsic birefringence of fluoride crystal lenses, stress-induced birefringence, covering the optical elements with antireflection coatings of lenses or mirror coatings.

The distribution of the optical path differences ΔOPL (α_(R), θ_(R)) is used to determine the effective birefringence distribution of the compensation coating that is applied to an optical element with an element axis. Examples of optical elements that are used are refractive or diffractive lenses, planar-parallel plates or mirrors. The optical surfaces of the optical element are defined as the areas that perform an optical function, that is normally the front and back surfaces. The element axis is provided for example by an axis of symmetry of a rotationally symmetrical lens. If the lens has no axis of symmetry, the element axis can be defined by the center of an incident bundle of rays or by a straight line in relation to which the ray angles of all light rays within the lens are minimal. The effective birefringence values depend on azimuth angles α_(F) that are based on a reference direction perpendicular to the element axis and on aperture angles θ_(F) that are based on the element axis.

A value pair (α_(F), θ_(F)) relating to the optical element has its corresponding counterpart in a value pair (α_(R), θ_(R)) of a ray in the image plane.

The effective birefringence distribution of the compensation coating is then determined in such a manner that the distribution of the optical path differences for two mutually orthogonal states of linear polarization are significantly reduced for the overall system including the compensation coating in comparison with the distribution without the compensation coating.

It is possible to influence the effective birefringence distribution through the selection of the material, the thickness profiles and the vapor-depositing angles for the individual layers of the compensation coating. The design of the coating and the process parameters are obtained here by using computer programs for coating design that determine the thickness profiles of the individual coatings and the process parameters from the effective birefringence distribution, the specified materials and the geometry of the optical element.

The compensation coating can in this case also be applied to more than one optical element. This increases the degrees of freedom in determining the compensation layers which, in addition to their compensating function, should also ensure a high degree of transmissivity of the coating.

Typical distributions of the optical path differences ΔOPL (α_(R), θ_(R)) for two mutually orthogonal states of linear polarization have small path differences for the aperture angle θ_(R)=0°. It is therefore beneficial if the birefringent effect of the compensation coating for the aperture angle θ_(F)=0° is close to zero. This is achieved if no large vapor-depositing angles are used in producing the compensation coating. It is therefore advantageous if the optical surface of the optical element to which the compensation coating is applied has the least possible curvature.

It is also possible that the compensation coating has a birefringence distribution with locally varying birefringence, so that regions with greater or lesser phase splitting are produced. The variation of the birefringence here comprises both variations in the absolute value of the phase splitting and variations in the direction, that is a directional dependence, which is given for example by the alignment of the principal axes which describe the birefringence effect. The birefringence distribution may for example be rotationally symmetrical to an element axis of the element provided with the coating. For example, a birefringence increasing or decreasing in a defined manner in the radial direction, that is from the center to the edge of the optical element, may be provided here. By selective control of the radial birefringence distribution, the compensation effect can be optimally adapted to different surface curvatures of the coated surfaces.

It is also possible that the birefringence distribution is not rotationally symmetrical. It may for example have an azimuthal modulation of the intensity of the birefringence, in particular with a birefringence distribution which has multiple radial symmetry, in particular 2-fold, 3-fold, 4-fold or 6-fold symmetry, with respect to the element axis. This allows for example at least partial compensation of an azimuthal modulation of the birefringence properties of a substrate having intrinsic birefringence, for example a substrate of <110>, <111> or <100>-oriented fluoride single crystal.

A coating of at least one optical surface of an optical component of an optical system may also be formed as an anisotropic coating and serve for example as a compensation coating. Elements with “anisotropic” coating may be useful independently of the other features of the invention and are explained in more detail below.

By rotating lenses of (100)- or (111)-orientation with respect to one another, approximately rotationally symmetrical distribution of the optical path differences ΔOPL (α_(R), θ_(R)) in the image plane is obtained as described above, the distribution depending only on the aperture angle θ_(R). The optical path differences can be further reduced by an optical element being provided with a compensation coating that has an effective birefringence distribution primarily depending only on the aperture angle θ_(F). This is achieved through homogenous thicknesses of the individual layers of the compensation coating over the optical element without variable thickness profiles.

The invention can be advantageously used by designing the optical element with the compensation coating as an interchangeable element.

The optical element nearest to the image plane is advantageously used for this.

In the process of realizing this concept, a first step comprises determining the distribution of the optical path differences ΔOPL (α_(R), θ_(R)) for two mutually orthogonal states of linear polarization for a bundle of rays in the image plane. In this step, the influence of all optical elements of the objective, including the coatings, is taken into account. The optical element to which the compensation coating is applied in a subsequent step is likewise in the path of rays of the bundle of rays during this step.

In a second step, the method described above is used to determine the effective birefringence distribution of a compensation coating and the resulting thickness profiles of the individual layers as well as the process parameters for producing the individual layers. In a third step, the optical element is removed from the path of rays and covered with the compensation coating. If the optical surface of the optical element is already covered with a coating, the existing coating is removed before the new coating is applied.

In a fourth step, the optical element with the compensation coating is returned to its original location within the objective.

The preferred material for the lenses in projection objectives is calcium fluoride, because calcium fluoride used together with quartz is particularly well-suited for color correction at a working wavelength of 193 nm and has an adequate transmissivity at a working wavelength of 157 nm. But the same can also be said for the fluoride crystals strontium fluoride or barium fluoride, because they are crystals of the same cubic crystal type.

The detrimental influence of the intrinsic birefringence is particularly noticeable if the light rays have large aperture angles within the lenses. This is the case for projection objectives which have a numerical aperture on the image side which is greater than 0.7, in particular greater than 0.8.

The intrinsic birefringence increases significantly with decreasing working wavelength. Therefore, in comparison to a wavelength of 248 nm, the intrinsic birefringence is more than twice as large at a wavelength of 193 nm and more than five times as large at a wavelength of 157 nm. The invention can therefore be used to particular advantage if the light rays have wavelengths of less than 200 nm, in particular less than 160 nm.

The objective may be a purely refractive projection objective comprising a multiplicity of lenses arranged with rotational symmetry about the optical axis, or it may be a projection objective of the catadioptric type of objectives.

Projection objectives of this type can be used advantageously in microlithography projection-exposure installations in which the light source is followed by an illuminating system, a mask-positioning system, a structure-bearing mask, a projection objective, an object-positioning system, and a light-sensitive substrate.

This microlithography projection-exposure installation can serve for producing micro-structured semiconductor components.

The invention also provides a suitable method for producing objectives. According to the method, lenses or lens parts of fluoride crystal whose lens axes point in a principal crystallographic direction are arranged such that they are rotated about the lens axes in such a way that the distribution ΔOPL (α_(R), θ_(R)) has significantly reduced values in comparison with a lens arrangement in which the lens axes of the fluoride crystal lenses point in the same principal crystallographic direction and in which the lenses are arranged with the same orientation.

The method further envisages forming groups with (100)-lenses and groups with (111)-lenses or (110)-lenses and using the groups in parallel. The method is used for example in the case of a projection objective comprising at least two fluoride crystal lenses in <100>-orientation and at least two lenses in <111>-orientation. The position of the reference directions is also known for these lenses. The method makes use of the inventive concept that the maximum values of the distribution ΔOPL (α_(R), θ_(R)) of the optical path differences can be significantly reduced by rotating the fluoride crystal lenses about the optical axis. Using suitable simulation methods, a bundle of rays originating from an object point is propagated through a projection objective and, based on the known optical properties of the fluoride crystal lenses, the distribution ΔOPL (α_(R), θ_(R)) in the image plane is determined. In an optimizing step, the angles of rotation between the fluoride crystal lenses are varied until the birefringence has tolerable values. The optimizing step can also take into account further constraints, such as for example the compensation of non-rotationally symmetrical lens errors by rotating the lenses. This optimizing step allows the maximum value of the distribution ΔOPL (α_(R), θ_(R)) to be reduced by up to 30%, in particular up to 50%, in comparison with a projection objective in which the fluoride crystal lenses are arranged with the same orientation. The optimization procedure may also include an intermediate step. In this intermediate step, the fluoride crystal lenses are assigned to groups in such a manner that, with equal orientation of the lenses, the lenses of a group produce for an outermost aperture ray a similar optical path difference between two mutually orthogonal states of linear polarization. In the subsequent optimizing step, the lenses are then rotated only within the groups in order to reduce the optical path differences. So, the (100)-lenses may be rotated first in such a way that the optical path differences caused by the (100)-lenses are reduced, and then the (111)-lenses are rotated in such a way that the optical path differences caused by the (111)-lenses are reduced. The distribution of the fluoride crystal lenses between lenses with (100)-orientation and (111)-orientation must take place during the optimization in such a way that the resulting (100)-distribution ΔOPL₁₀₀ (α_(R), θ_(R)) and the resulting (111)-distribution ΔOPL₁₁₁ (α_(R), θ_(R)) compensate one another to a large extent. A corresponding statement also applies to the parallel use of (100)-lenses and (110)-lenses.

The invention also relates to a method of manufacturing a lens in which, in a first step, a plurality of plates of fluoride crystal are joined in an optically seamless manner to form a blank and, in a second step, the lens is produced from the blank through known manufacturing methods. The plates are in this case arranged such that they are rotated with respect to one another about the surface normals in the same manner as described above for lenses or lens parts.

Plates whose surface normals point in the same principal crystallographic direction or in a principal crystallographic direction equivalent thereto advantageously have the same axial thickness.

If (100)-plates are joined in an optically seamless manner with (111)-plates, the ratio between the sum of the thicknesses of the (111)-plates and the sum of the thicknesses of the (100)-plates should be 1.5±0.2.

If (100)-plates are joined in an optically seamless manner with (110)-plates, the ratio between the sum of the thicknesses of the (110)-plates and the sum of the thicknesses of the (100)-plates should be 4.0±0.4.

The invention also relates to optical components with at least one “anisotropic” coating. In particular, a coating of at least one optical surface of an optical component of an optical system may be formed as an anisotropic coating and serve for example as a compensation coating. For the purposes of this application, an “anisotropic” coating is a coating which has a pronounced directional dependence of its optical effect on the direction of the electrical field vectors of the incident radiation in the area of incidence. In a region considered, an anisotropic coating may consequently have a fast or slow axis, which corresponds to a preferred direction of the coating.

Anisotropic thin films with special microstructures, which can be influenced by the production conditions, and resultant anisotropic properties are known per se. The article “Play the angles to create exotic thin films” by M. Seto and M. Brett in: Vacuum Solutions, March/April 2000, pages 26-31, describes the morphology of various thin films which can be produced by vapor deposition at high depositing angles (glancing angle deposition, GLAD). Such porous thin films, which often have columnar structures with features dependent on the coating direction, may also be useful for optical applications. Example of polarization elements of this type are specified inter alia in the article “Review of birefringent and chiral optical interference coatings” by I. Hodgkinson and Q. H. Wu in OIC 2000/2001, pages 1 and 2 or in “Birefringent Thin Film Polarizing Elements”, by I. Hodgkinson and Q. H. Wu, World Scientific, Singapore, New Jersey, London, Hong Kong, ISBN 981-02-2906-2, the content of which is made the content of this description by reference.

Particularly suitable for compensation are anisotropic coatings with a local variation of the anisotropy. The variation may comprise the direction of the preferred direction and/or the absolute amount of the phase splitting produced by the coating.

To produce coatings with a birefringence distribution with a locally varying birefringence, all known coating methods can be used with suitable modification of the process control, in particular PVD processes, such as electron beam evaporation, or sputtering. When producing coatings that are at least anisotropic in certain regions, in a preferred embodiment the coating material is applied to at least one region of the substrate surface or an already existing coating at an angle of coverage, in particular a vapor-depositing angle, which is so large that an anisotropic coating structure is produced. For this purpose, it is possible, on the basis of known vapor-depositing installations, for example to reduce significantly the difference in height between a material source and the substrates, in order to achieve an inclined vapor deposition of coating material, for which typical vapor-depositing angles can lie in the range of 30° to 40° or more. Here, the angle between the direction of incidence of a coating material on the substrate surface and the surface normal of the substrate surface at the coating location is understood as the vapor-depositing angle (angle of coverage).

It has been found that it is possible by suitable modifications of known diaphragm methods to produce anisotropic coatings with a defined distribution of their birefringence properties, in particular with a defined distribution of the layer anisotropy, on flat, slightly or greatly curved optical substrates. In a variant, the following steps are carried out to control the birefringence distribution and/or the anisotropy of the coating. A rotation of the substrate about a substrate axis of rotation is produced. Preferably used for this purpose is a planetary system, in which each substrate is arranged on a substrate carrier which performs a rotation of its own about its substrate axis of rotation and a global rotation about a main axis of rotation of the planetary system. In this installation, the substrate surface is then coated with the material of a material source at large coating angles. In this case, a periodic shading of the coating material takes place during the rotation of the substrate to produce a coating time that is dependent on the radial position of the coating location, according to a prescribed radial time profile. In this case, the shading may be carried out by one or more diaphragms in such a way that small coating angles (for example <30° to 35°) are shaded, so that the material meets the substrate surface exclusively or at least predominantly at very large vapor-depositing angles (for example 40° or more) from selected directions. By suitable shaping of the diaphragms, any desired radial-symmetrical profiles of the birefringence distribution with different degrees of anisotropy can be produced.

The invention also relates to a method for producing a polarization-optical element, i.e. an optical component or arrangement with a defined effect on the state of polarization of the incident radiation, which may also be useful and able to be protected independently of the other features of the invention. The polarization-optical element may be, for example, a retardation element (retarder). The method envisages changing of the local birefringence distribution of a coating after completion of the coating operation. This changing may be carried out by local exposure of the finished coating to an energy suitable for changing the morphology of the coating according to a prescribed spatial distribution. Subsequent local changing of the layer properties may be provided in interference layer systems of all kinds (for example reflection layers, antireflection layers). It is particularly effective in the case of anisotropic coatings, since the morphologies responsible for the anisotropy are typically non-equilibrium structures with limited stability. The definition of the regions to be exposed to energy may take place for example with the aid of one or more masks. In particular, the morphology of a coating may be changed by thermal exposure. This is possible for example by irradiation with an infrared laser or some other type of irradiation producing adequate thermal energy. Working with an electron beam, possibly with a writing effect, is also possible. Alternatively or in addition to introducing energy thermally, the energy may also be applied mechanically, for example with the aid of ion beams and/or heating dies. As a particular advantage, the subsequent working of the layer structure allows non-rotationally-symmetrical birefringence distributions to be set, with a very small-scale setting of properties possibly being possible, for example with typical range sizes in the mm or cm range.

For example, layers vapor-deposited at low coating temperatures (cold), in particular anisotropic layer systems, may be changed locally in their birefringence properties by laser radiation. In this way, a polarization filter with specific local modulation of the phase splitting can be produced.

The subsequent changing of the birefringence distribution also allows changes of its polarization-optical behavior to be performed on ready-assembled optical systems, for example projection objectives for microlithography. For this purpose, the optical system may firstly be assembled using at least one component with an anisotropic coating, or some other non-equilibrium coating, and measured. The measurement result can be used to determine a desired effective birefringence distribution of the at least one compensation coating, which is required to adjust the system polarization-optically. Then, the optical element provided with the coating can be disassembled, in order to perform the subsequent changes of the layer properties locally by suitable introduction of energy. After installing the optical element changed in this way, the optical system then has the desired properties. The invention consequently also relates to a special production method for an optical system, in particular an optical system for microlithography.

The invention is explained in more detail on the basis of the drawings.

FIG. 1 shows a section through a fluoride crystal block perpendicular to the {100} crystallographic planes together with a lens of a projection objective, in schematic representation;

FIGS. 2A-C each show a planar-parallel (100)-, (111)- and (110)-lens in a schematic three-dimensional representation;

FIG. 3 shows a system of coordinates for defining the aperture angle and the azimuth angle;

FIGS. 4A-F show the birefringence distribution for (100)-lenses in various representations, and the birefringence distribution for two (100)-lenses rotated by 45° with respect to one another;

FIGS. 5A-F show the birefringence distribution for (111)-lenses in various representations, and the birefringence distribution for two (111)-lenses rotated by 60° with respect to one another;

FIGS. 6A-G show the birefringence distribution for (111)-lenses in various representations, and the birefringence distribution for two (110)-lenses rotated by 90° with respect to one another, and respectively for four (110)-lenses rotated by 45° with respect to one another;

FIG. 7 shows the lens section of a refractive projection objective;

FIG. 8 shows the lens section of a catadioptric projection objective;

FIG. 9 shows a microlithography projection-exposure installation in a schematic representation;

FIG. 10 is a diagram which shows the dependence of the phase splitting ΔPH caused by birefringence on the direction of irradiation for a homogeneous compensation coating and for an anisotropic compensation coating;

FIG. 11 is a diagram which shows the dependence of the phase splitting ΔPH caused by birefringence on the direction of irradiation for layers with different numbers of individual layers and different production conditions;

FIG. 12 schematically shows the production of an anisotropic coating on a lens in a coating installation with a planetary system;

FIG. 13 schematically shows shading geometries for the coating production with the aid of shading diaphragms; and

FIG. 14 schematically shows a production method for a polarizer with subsequent changing of the properties of an anisotropic coating.

FIG. 1 schematically shows a section through a fluoride crystal block 3. The section is chosen such that the {100} crystallographic planes 5 appear as individual lines, so that the {100} crystallographic planes 5 are perpendicular to the plane of the paper. The fluoride crystal block 3 serves as a blank or starting material for the (100)-lens 1. In this example, the (100)-lens 1 is a bi-convex lens with the lens axis EA, which is at the same time the axis of symmetry of the lens. The lens 1 is formed out of the fluoride crystal block in such a manner that the lens axis EA is perpendicular to the {100} crystallographic planes.

FIG. 2A illustrates by a three-dimensional representation how the intrinsic birefringence is associated with the crystallographic directions in the case where the lens axis EA points in the <100> crystallographic direction. A circular planar-parallel plate 201 of calcium fluoride is represented. The lens axis EA points in the <100> crystallographic direction. In addition to the <100> crystallographic direction, the crystallographic directions <101>, <1 10>, <10 1> and <110> are likewise represented by arrows. The intrinsic birefringence is schematically represented by four “paddles” 203, whose surface areas indicate the amount of intrinsic birefringence for the respective direction of a light ray. The maximum intrinsic birefringence occurs in the crystallographic directions <101>, <1 10>, <10 1> and <110>, i.e., for light rays which have inside the lens an aperture angle of 45° an azimuth angle of 0°, 90°, 180° and 270°, respectively. Minimum values of the intrinsic birefringence occur for azimuth angles of 45°, 135°, 225° and 315°. The intrinsic birefringence becomes zero for an aperture angle of 0°.

FIG. 2B illustrates by a three-dimensional representation how the intrinsic birefringence is associated with the crystallographic directions in the case where the lens axis DA points in the <111> crystallographic direction. A circular planar-parallel plate 205 of calcium fluoride is represented. The lens axis EA points in the <111> crystallographic direction. In addition to the <111> crystallographic direction, the crystallographic directions <011>, <101>, and <110> are likewise represented by arrows. The intrinsic birefringence is schematically represented by three “paddles” 207, whose surface areas indicate the amount of intrinsic birefringence for the respective direction of a light ray. The maximum intrinsic birefringence respectively occurs in the crystallographic directions <011>, <101>, and <110>, i.e. for light rays which have inside the lens an aperture angle of 35° and an azimuth angle of 0°, 120°, and 240°, respectively. Minimum values of the intrinsic birefringence occur for azimuth angles of 60°, 180°, and 300°. The intrinsic birefringence becomes zero for an aperture angle of 0°.

FIG. 2C illustrates by a three-dimensional representation how the intrinsic birefringence is associated with the crystallographic directions in the case where the lens axis EA points in the <110> crystallographic direction. A circular planar-parallel plate 209 of calcium fluoride is represented. The lens axis EA points in the <110> crystallographic direction. In addition to the <110> crystallographic direction, the crystallographic directions <01 1>, <10 1>, <101>, and <011> are likewise represented by arrows. The intrinsic birefringence is schematically represented by five “paddles” 211, whose surface areas indicate the amount of intrinsic birefringence for the respective direction of a light ray. The maximum intrinsic birefringence respectively occurs on the one hand in the direction of the lens axis EA, and on the other hand respectively in the crystallographic directions <01 1>, <10 1>, <101>, and <011>, i.e. for light rays with an aperture angle of 0°, or respectively with an aperture angle of 60° and the four azimuth angles obtained by projection of the <01 1>, <10 1>, <101>, and <011> crystallographic directions into the {110} crystallographic plane. However, aperture angles of this magnitude do not occur in crystal material, because the maximum aperture angles are limited to less than 45° by the refractive index of the crystal.

FIG. 3 illustrates how the aperture angle θ and the azimuth angle α are defined. For the (100)-lens of FIG. 2, the z-axis points in the <100> crystallographic direction, the x-axis points in the direction that is obtained by projecting the <110> crystallographic direction into the {100} crystallographic plane. The z-axis is in this case the same as the lens axis and the x-axis is the same as the reference direction.

It is known from the cited Internet publication that measurements for calcium fluoride gave a birefringence value of (6.5±0.4) nm/cm for light with a wavelength of λ=156.1 nm traveling in the <110> crystallographic direction. Using this measured value as a normalizing factor, the birefringence distribution Δn (θ, α) of a calcium fluoride lens can be calculated theoretically as a function of the crystal orientation. The calculation is based on the formalisms that are known from crystal optics for the calculation of index ellipsoids as a function of the direction of a light ray. The theoretical principles may be found for example in “Lexikon der Optik”, Spektrum Akademischer Verlag Heidelberg Berlin, 1999 under the keyword “Kristalloptik”.

In more recent measurements carried out by the applicant, a birefringence value of 11 nm/cm was found for light with a wavelength of λ=156.1 nm traveling in the <110> crystallographic direction in a calcium fluoride crystal. The statements made hereafter for the normalizing factor Δn_(max)=6.5 nm/cm can be converted without difficulty to the normalizing factor Δn_(max)11 nm/cm.

In FIG. 4A, the amount of the intrinsic birefringence is represented as a function of the aperture angle θ for the azimuth angle α=0° for a (100)-lens. The value of 6.5 nm/cm for the intrinsic birefringence at an aperture angle of θ=45° corresponds to the measured value. The curve profile was determined on the basis of the formulas that are known from crystal optics.

In FIG. 4B, the amount of the intrinsic birefringence is represented as a function of the azimuth angle α for the aperture angle θ=45° for a (100)-lens. The fourfold azimuthal symmetry is readily apparent.

In FIG. 4C, the birefringence distribution Δn (θ, α) is represented for individual light ray directions in the (θ, α)-angular domain for a (100)-lens. Each line represents the magnitude and direction for a light ray direction defined by the aperture angle θ and the azimuth angle α. The length of the lines is in proportion to the amount of the birefringence, or the difference between the lengths of the principal axes of the intersecting ellipse, while the direction of the lines indicates the orientation of the longer principal axis of the intersecting ellipse. The intersecting ellipse is obtained as the intersection between the index ellipsoid for the ray in the direction (θ, α) and a plane which is perpendicular to the direction of the light ray and passes through the center of the index ellipsoid. Both the directions and the lengths of the lines are indicative of the four-fold symmetry of the distribution. The length of the lines, and consequently the birefringence, is largest at the azimuth angles of 0°, 90°, 180° and 27°.

FIG. 4D then shows the birefringence distribution Δn (θ, α) that is obtained if two adjacent planar-parallel (100)-lenses of equal thickness are arranged such that they are rotated by 45°. The resulting birefringence distribution Δn (θ, α) is independent of the azimuth angle α. The longer principal axes of the intersecting ellipses run tangentially. The resultant optical path differences of two mutually orthogonal states of polarization are obtained by multiplying the birefringence values by the physical path lengths traveled by the light rays inside the planar-parallel (100)-lenses. Rotationally symmetrical birefringence distributions are obtained by arranging n planar-parallel (100)-lenses of equal thickness in such a manner that the angles of rotation β between respective pairs of lenses conform to the equation ${\beta = {\frac{90{^\circ}}{n} + {{{m \cdot 90}{^\circ}} \pm {5{^\circ}}}}},$ where n stands for the number of the planar-parallel (100)-lenses and m is an integral number. In comparison with a uniformly oriented arrangement of the lenses, the maximum value of the birefringence for the aperture angle θ=30° can be reduced by 30%. An approximately rotationally symmetrical distribution of the optical path differences for two mutually orthogonal states of linear polarization is also obtained with lenses of an arbitrary shape if all rays of a bundle have angles of similar magnitude and cover light paths of similar length inside the lenses. The lenses should therefore be assembled into groups in such a manner that the rays satisfy the condition specified above as well as possible.

In FIG. 4E, the amount of the intrinsic birefringence is represented as a function of the aperture angle .θ for the azimuth angle α=0° for the two adjacent planar-parallel (100)-lenses of equal thickness of FIG. 4D. The maximum value for the intrinsic birefringence at the aperture angle θ=41° is 4.2 nm/cm and is consequently reduced by 35% compared to the maximum value of 6.5 nm/cm in FIG. 4A.

In FIG. 4F, the amount of the intrinsic birefringence is represented as a function of the azimuth angle α for the aperture angle θ=41° for the two adjacent planar-parallel (100)-lenses of equal thickness of FIG. 4D. The intrinsic birefringence is independent of the azimuth angle α.

In FIG. 5A, the amount of the intrinsic birefringence is represented as a function of the aperture angle θ for the azimuth angle α=0° for a (111)-lens. The value of 6.5 nm/cm for the intrinsic birefringence at an aperture angle of θ=35° corresponds to the measured value. The curve profile was determined on the basis of the formulas that are known from crystal optics.

In FIG. 5B, the amount of the intrinsic birefringence is represented as a function of the azimuth angle α for the aperture angle θ=35° for a (111)-lens. The threefold azimuthal symmetry is readily apparent.

FIG. 5C shows the birefringence distribution Δn (θ, α) for individual light ray directions in the (θ, α)-angular domain for a (111)-lens in the same form as that already represented by FIG. 4C. Both the directions and the lengths of the lines are indicative of the threefold symmetry of the distribution. The length of the lines, and consequently the birefringence, is largest at the azimuth angles of 0°, 120°, and 240°. By contrast with a (100)-lens, the orientation of the birefringence turns by 90° when a light ray passes through a lens with an azimuth angle of 180° instead of with an azimuth angle of 0°. Consequently, the birefringence can be compensated, for example, by two (111)-lenses of the same orientation if the ray angles of a bundle of rays change over their signs between the two lenses.

FIG. 5D then shows the birefringence distribution Δn (θ, α) that is obtained if two adjacent planar-parallel (111)-lenses of equal thickness are arranged such that they are rotated by 60°. The resulting birefringence distribution Δn (θ, α) is independent of the azimuth angle α. However, by contrast with FIG. 4C, the longer principal axes of the intersecting ellipses run radially. The resultant optical path differences of two mutually orthogonal states of polarization are obtained by multiplying the birefringence values by the physical path lengths traveled by the light rays inside the (111)-lenses. Rotationally symmetrical birefringence distributions are likewise obtained by arranging n planar-parallel (111)-lenses of equal thickness in such a manner that the angles of rotation between respective pairs of lenses conform to the equation ${\gamma = {\frac{120{^\circ}}{k} + {{{l \cdot 120}{^\circ}} \pm {5{^\circ}}}}},$ where k stands for the number of the planar-parallel (111)-lenses and 1 is an integral number. In comparison with a uniformly oriented arrangement of the lenses, the value of the birefringence for the aperture angle θ=30° can be reduced by 68%. An approximately rotationally symmetrical distribution of the optical path differences for two mutually orthogonal states of linear polarization is also obtained with lenses of an arbitrary shape if all rays of a bundle in the lenses have angles of similar magnitude and cover light paths of similar length inside the lenses. The lenses should therefore be assembled into groups in such a manner that the rays satisfy the condition specified above as well as possible.

In FIG. 5E, the amount of the intrinsic birefringence is represented as a function of the aperture angle .θfor the azimuth angle α=0° for the two adjacent planar-parallel (111)-lenses of equal thickness of FIG. 5D. The maximum value for the intrinsic birefringence at the aperture angle θ=41° is 2.8 nm/cm and is consequently reduced by 57% compared to the maximum value of 6.5 nm/cm in FIG. 5A.

In FIG. 5F, the amount of the intrinsic birefringence is represented as a function of the azimuth angle α for the aperture angle θ=41° for the two adjacent planar-parallel (111)-lenses of equal thickness of FIG. 5D. The intrinsic birefringence is independent of the azimuth angle α.

By combining groups with (100)-lenses and groups with (111)-lenses within a projection objective, the optical path differences produced by these lenses for two mutually orthogonal states of linear polarization can be compensated to a large extent. This requires that an approximately rotationally symmetrical distribution of the optical path differences is first obtained within these groups by rotating the lenses and that the two distributions of optical path differences are then compensated by combining a group with (100)-lenses and a group with (111)-lenses. The compensation takes advantage of the fact that the orientations of the longer principal axes of the intersecting ellipses for the birefringence distribution of a group with rotated (100)-lenses is perpendicular to the orientations of the longer principal axes of the intersecting ellipses for the birefringence distribution of a group with rotated (111)-lenses, as can be seen in FIGS. 4D and 5D. It is of importance that on the one hand the individual groups generate an approximately rotationally symmetrical distribution of the optical path differences, and on the other hand that the sum of the contributions of the groups with (100)-lenses is of approximately equal absolute magnitude as the sum of the contributions of the groups with (111)-lenses.

In FIG. 6A, the amount of the intrinsic birefringence is represented as a function of the aperture angle θ for the azimuth angle α=0° for a (110)-lens. The value of 6.5 nm/cm for the intrinsic birefringence at an aperture angle of θ=0° corresponds to the measured value. The curve profile was determined on the basis of the formulas that are known from crystal optics.

In FIG. 6B, the amount of the intrinsic birefringence is represented as a function of the azimuth angle α for the aperture angle θ=35° for a (110)-lens. The twofold azimuthal symmetry is readily apparent.

FIG. 6C shows the birefringence distribution Δn (θ, α) for individual light ray directions in the (θ, α)-angular domain for a (110)-lens in the same form as that already represented by FIG. 4C. Both the directions and the lengths of the lines are indicative of the twofold symmetry of the distribution. The line with a maximum length, and consequently the maximum birefringence, is obtained for the aperture angle θ=0°.

FIG. 6D then shows the birefringence distribution Δn (θ, α) that is obtained if two adjacent planar-parallel (110)-lenses of equal thickness are arranged such that they are rotated by 90°.

The resulting birefringence distribution Δn (θ, α) has a fourfold azimuthal symmetry. Maximum birefringence values occur at the azimuth angles α=45°, 135°, 225° and 315°, with a birefringence value of 2.6 nm/cm for the aperture angle θ=40°.

FIG. 6E then shows the birefringence distribution Δn (θ, α) that is obtained when the two planar-parallel (110)-lenses of equal thickness of FIG. 6C are combined with two further planar-parallel (110)-lenses of equal thickness. The angle of rotation between pairs of the (110)-lenses is 45°. The resultant birefringence distribution Δn (θ, α) is independent of the azimuth angle α. However, by contrast with FIG. 4C, the longer principal axes of the intersecting ellipses extend radially, that is similar to the distribution of FIG. 5C. The resultant optical path differences of two mutually orthogonal states of polarization are obtained by multiplying the birefringence values by the physical path lengths traveled by the light rays inside the (110)-lenses. Rotationally symmetrical birefringence distributions are likewise obtained by arranging 4·n planar-parallel (110)-lenses of equal thickness in such a manner that the angles of rotation β between respective pairs of the lenses conforms to the equation ${\beta = {\frac{45{^\circ}}{n} + {{m\quad 90{^\circ}} \pm {5{^\circ}}}}},$ where 4·n stands for the number of planar-parallel (100)-lenses and m is an integral number. An approximately rotationally symmetrical distribution of the optical path differences for two mutually orthogonal states of linear polarization is also obtained with lenses of an arbitrary shape if all rays of a bundle in the lenses have angles of similar magnitude and cover light paths of similar length inside the lenses. The lenses should therefore be assembled into groups in such a manner that the rays satisfy the condition specified above as well as possible.

FIG. 6F shows the amount of the intrinsic birefringence as a function of the aperture angle θ for the azimuth angle α=0° for the four adjacent planar-parallel (110)-lenses of equal thickness of FIG. 6E. The value for the intrinsic birefringence at the aperture angle θ=41° is in this case 1.0 nm/cm and is consequently reduced by 84% in comparison with the maximum value of 6.5 nm/cm in FIG. 5A.

In FIG. 6G, the amount of the intrinsic birefringence is represented as a function of the azimuth angle α for the aperture angle θ=41° for the four adjacent planar-parallel (110)-lenses of equal thickness of FIG. 6E. The intrinsic birefringence is independent of the azimuth angle α.

By combining groups with (110)-lenses and groups with (100)-lenses within a projection objective, the optical path differences produced by these lenses for two mutually orthogonal states of linear polarization can be compensated to a large extent. This requires that an approximately rotationally symmetrical distribution of the optical path differences is first obtained within these groups by rotating the lenses and that the two distributions of optical path differences are then compensated by combining a group with (110)-lenses and a group with (100)-lenses. The compensation takes advantage of the fact that the orientations of the longer principal axes of the intersecting ellipses for the birefringence distribution of a group with rotated (110)-lenses is perpendicular to the orientations of the longer principal axes of the intersecting ellipses for the birefringence distribution of a group with rotated (100)-lenses, as can be seen in FIGS. 4D and 6E. It is of importance that on the one hand the individual groups generate an approximately rotationally symmetrical distribution of the optical path differences, and on the other hand that the sum of the contributions of the groups with (110)-lenses is of approximately equal absolute magnitude as the sum of the contributions of the groups with (100)-lenses.

In FIG. 7, the lens section of a refractive projection objective 611 is represented for the wavelength of 157 nm. The optical data for this objective are listed in Table 1. The exemplary embodiment is taken from the applicant's patent application PCT/EP00/13148, where it corresponds to FIG. 7 and Table 6. For a more detailed functional description of the objective, the reader is referred to the patent application PCT/EP00/13148. All lenses of this objective consist of calcium fluoride crystal. The numerical aperture on the image side of the objective is 0.9. The imaging performance of this objective is corrected so well that the wavefront deviation from an ideal spherical wave is smaller 1.8 mλ on the basis of the wavelength of 157 nm. Especially in these high-performance objectives, it is necessary to reduce the detrimental influences, such as of intrinsic birefringence, as much as possible.

For the exemplary embodiment of FIG. 6, the aperture angles θ and path lengths RL_(L) of the outermost aperture ray 609 were calculated for the individual lenses L601 to L630. The outermost aperture ray 609 originates from the object point at the coordinates x=0 mm and y=0 mm and has in the image plane an angle relative to the optical axis that corresponds to the numerical aperture on the image side. The outermost aperture ray 609 is used because its aperture angle inside the lenses is close to the maximum aperture angle. TABLE 2 Optical path Optical path Optical path Optical path difference difference difference difference Aperture Path length (111)-lens (111)-lens (100)-lens (100)-lens Lens angle θ [°] RL_(L) [mm] α_(L) = 0° [nm] α_(L) = 60° [nm] α_(L) = 0° [nm] α_(L) = 45° [nm] L601 8.1 15.1 2.9 −2.2 −0.8 −0.4 L602 8.7 8.2 1.7 −1.2 −0.5 −0.2 L603 7.8 9.5 1.7 −1.3 −0.4 −0.2 L604 10.7 7.2 1.9 −1.3 −0.6 −0.3 L605 9.4 6.5 1.5 −1.0 −0.4 −0.2 L606 10.3 8.5 2.1 −1.4 −0.7 −0.3 L607 21.8 12.7 6.6 −2.7 −3.9 −1.8 L608 25.4 22.2 12.8 −4.4 −8.7 −3.9 L609 16.3 36.1 14.3 −7.6 −6.8 −3.3 L610 12.2 15.2 4.5 −2.9 −1.7 −0.8 L611 2.3 26.6 1.4 −1.3 −0.1 −0.1 L612 2.3 32.2 1.6 −1.5 −0.1 −0.1 L613 −18.3 30.4 −6.6 13.5 −7.0 −3.3 L614 −18.7 22.0 −4.8 10.0 −5.3 −2.5 L615 −14.0 10.2 −2.0 3.5 −1.5 −0.7 L616 −1.3 29.8 −0.8 0.9 0.0 0.0 L617 26.4 31.6 18.6 −6.1 −13.0 −5.7 L618 33.5 14.3 9.3 −2.0 −7.9 −3.1 L619 26.5 7.5 4.4 −1.4 −3.1 −1.4 L620 19.3 6.4 3.0 −1.4 −1.6 −0.8 L621 6.7 8.0 1.3 −1.0 −0.3 −0.1 L622 −10.3 7.7 −1.3 1.9 −0.6 −0.3 L623 −11.9 9.6 −1.8 2.8 −1.0 −0.5 L624 0.3 17.8 0.1 −0.1 0.0 0.0 L625 6.0 16.3 2.3 −1.8 −0.5 −0.2 L626 −24.0 9.0 −1.9 5.0 −3.2 −1.5 L627 −35.6 8.0 −0.9 5.2 −4.7 −1.7 L628 −39.4 12.0 −1.0 7.6 −7.5 −2.5 L629 −35.3 27.3 −3.3 17.7 −15.7 −5.9 L630 −35.3 26.0 −3.1 16.9 −15.0 −5.6 Sum 64.5 42.3 112.9 47.4 Optical path Optical path Optical path Optical path difference difference difference difference (110)-lens (110)-lens (110)-lens (110)-lens Lens α_(L) = 0° [nm] α_(L) = 45° [nm] α_(L) = 90° [nm] α_(L) = 135° [nm] L601 −9.0 −9.0 −9.1 −9.0 L602 −4.9 −4.8 −4.9 −4.8 L603 −5.7 −5.7 −5.7 −5.7 L604 −4.1 −4.1 −4.1 −4.1 L605 −3.8 −3.8 −3.8 −3.8 L606 −4.8 −4.8 −4.8 −4.8 L607 −4.2 −4.2 −4.3 −4.2 L608 −5.3 −5.7 −5.8 −5.7 L609 −16.5 −16.5 −16.7 −16.5 L610 −8.2 −8.2 −8.2 −8.2 L611 −17.2 −17.2 −17.2 −17.2 L612 −20.8 −20.8 −20.8 −20.8 L613 −12.5 −12.6 −12.7 −12.6 L614 −8.9 −8.9 −9.0 −8.9 L615 −5.1 −5.1 −5.2 −5.1 L616 −19.3 −19.3 −19.3 −19.3 L617 −6.7 −7.6 −7.5 −7.6 L618 −0.6 3.2 −1.4 3.2 L619 −1.6 −1.8 −1.8 −1.8 L620 −2.5 −2.5 −2.5 −2.5 L621 −4.9 −4.9 −4.9 −4.9 L622 −4.4 −4.4 −4.4 −4.4 L623 −5.2 −5.2 −5.2 −5.2 L624 −11.6 −11.6 −11.6 −11.6 L625 −9.9 −9.9 −10.0 −9.9 L626 −2.5 −2.6 −2.6 −2.6 L627 0.1 2.1 −0.5 2.1 L628 1.0 4.0 −0.3 4.0 L629 0.5 6.9 −1.9 6.9 L630 0.4 6.5 −1.9 6.5 Sum −198.2 −178.7 −208.0 −178.8

Table 2 lists not only the aperture angles θ and optical path lengths RL_(L) for the outermost aperture ray but also the optical path differences for two mutually orthogonal states of linear polarization for different lens orientations. The optical path differences are listed for (111)-lenses, (100)-lenses and (110)-lenses, the azimuth angle α_(L) of the outermost edge ray within the lenses being 0° and 60° for a (111)-lens, 0° and 45° for a (100)-lens and 0°, 45°, 90° and 135° for a (110)-lens.

According to Table 2, the aperture angles θ for the lenses L608, L617, L618, L619, L627, L628, L629 and L630 are larger than 25°, exceeding even 30° for the lenses L618, L627, L628, L629 and L630. Large aperture angles occur in particular in the lenses L627 to L630 which are closest to the image plane.

The design of the projection objective produced the result that the maximum aperture angle of all the light rays is less than 45°. The maximum aperture angle for the outermost aperture ray is 39.4° in the lens L628. It also proved helpful to use two thick planar lenses L629 and L630 immediately before the image plane.

The diameter of the diaphragm between the lenses L621 and L622 is 270 mm. The diameter of the lens L618 is 207 mm, and the diameters of the lenses L627 to L630 are all smaller than 190 mm. Consequently, the diameters of these lenses, which have large aperture angles, are smaller than 80% of the diaphragm diameter.

As can be concluded from Table 2, it is beneficial for individual lenses with large aperture angles to orient them in the (100)-direction, because the birefringence values are lower overall. This is due to the fact that the angles at which the influence of the <110> crystallographic directions begins to be noticeable are larger in (100)-lenses than in (111)-lenses. For example in the lenses L608, L609 and L617, the optical path differences are more than 30% lower.

The two planar-parallel lenses L629 and L630 are a good example to show how the birefringence can be significantly reduced by rotating the lenses with respect to one another. Both lenses have equal aperture angles for the outermost aperture ray of 35.3°, and similar path lengths of 27.3 mm and 26.0 mm, respectively. If the two lenses were installed with equal orientation as (100)-lenses, they would produce an optical path difference of 30.7 nm. By contrast, if the two (100)-lenses are rotated with respect to one another by 45°, the optical path difference is reduced to 20.9 nm, representing a reduction of 32%. If the two lenses were installed with equal orientation as (111)-lenses, they would produce an optical path difference of 34.6 nm. However, if the two (111)-lenses are rotated with respect to one another by 60°, the optical path difference is reduced to 13.6 nm, that is by 61%.

Almost complete compensation of the optical path differences due to the intrinsic birefringence caused by the lenses L629 and L630 for two mutually orthogonal states of linear polarization can be achieved by splitting the lens L629 into the lenses L6291 and L6292, and splitting the lens L630 into the lenses L6301 and L6302, wherein the lens L6291 is a (100)-lens with a thickness of 9.15 mm, the lens L6292 is a (111)-lens with a thickness of 13.11 mm, the lens L6301 is a (100)-lens with a thickness of 8.33 mm, and the lens L6302 is a (111)-lens with a thickness of 12.9 mm. The lenses L6291 and L6301 are rotated with respect to one another by 45°; the lenses L6292 and L6302 are rotated by 60°. The resultant maximum optical path difference is in this case then 0.2 nm. The lenses L6291 and L6292 as well as the lenses L6301 and L6302 can be joined in an optically seamless manner, for example by means of wringing. This principle can also be applied if the projection objective contains only one crystal lens. The one crystal lens in this case is split into at least two lenses that are arranged such that they are rotated in relation to one another. The lenses can be joined together by wringing. Another possibility is first to produce an optically seamless combination of individual plates that have the desired crystal orientation and then form the lens from the joined-together plates in a further process step.

Another possibility for reducing the detrimental influence of the intrinsic birefringence of the lenses L629 and L630 is to split the lens L629 into the lenses L6293 and L6294 and to split the lens L630 into the lenses L6303 and L6304, wherein the lens L6293 is then a (110)-lens with a thickness of 11.13 mm, the lens L6294 is a (110)-lens with a thickness of 11.13 mm, the lens L6303 is a (110)-lens with a thickness of 10.62 mm, and the lens L6304 is a (110)-lens with a thickness of 10.62 mm. The lenses L6293 and L6294, as well as the lenses L6303 and L6304 are in each case rotated with respect to one another by 90°, and the angle of rotation between the lenses L6293 and L6303 is 45°. The resultant maximum optical path difference is in this case 4.2 nm. The lenses L6293 and L6294 as well as the lenses L6303 and L6304 can be joined as lens parts in an optically seamless manner, for example by means of wringing.

Almost complete compensation of the optical path differences for two mutually orthogonal states of linear polarization caused by the highly loaded lenses L629 and L630 can be achieved by splitting each lens into three lens parts L6295, L6296 and L6297, and L6305, L6306 and L6307, respectively, wherein the lens L6295 is then a (100)-lens with a thickness of 4.45 mm, the lenses L6296 and L6297 are (110)-lenses with a thickness of 8.90 mm, the lens L6305 is a (100)-lens with a thickness of 4.25 mm, and the lenses L6306 and L6307 are (110)-lenses with a thickness of 8.49 mm. The lenses L6294 and L6304 are rotated with respect to one another by 45°; pairs of the lenses L6295, L6297, L6306 and L6307 are rotated with respect to one another by 45° In this combination, the resultant maximum optical path difference is reduced to below 0.1 nm. The lenses L6295 to L6297 as well as the lenses L6305 to L6307 can be joined as lens parts in an optically seamless manner, for example by means of wringing.

Another possibility for reducing the detrimental influence of the intrinsic birefringence by the lenses L629 and L630 is to combine two (110)-lenses with a (100)-lens. The two (110)-lenses are to be installed such that they are rotated by 90° with respect to one another, while the angle of rotation between the (100)-lens and the (110)-lenses is 45°+m·90°, where m is an integral number. For this purpose, the lens L629 is split into the lenses L6298 and L6299 and the lens L630 is split into the lenses L6308 and L6309, wherein the lens L6298 is then a (110)-lens with a thickness of 17.40 mm, the lens L6299 is a (110)-lens with a thickness of 4.87 mm, the lens L6308 is a (110)-lens with a thickness of 12.53 mm, and the lens L6309 is a (100)-lens with a thickness of 8.70 mm. The resultant maximum optical path difference is 3.1 nm. The lenses L6298 and L6299 as well as the lenses L6308 and L6309 can be joined as lens parts in an optically seamless manner, for example by means of wringing.

In FIG. 8, the lens section of a catadioptric projection objective 711 is represented for the wavelength of 157 nm. The optical data for this objective are listed in Table 3. The exemplary embodiment is taken from the applicant's patent application PCT/EP00/13148, where it corresponds to FIG. 9 and Table 8. For a more detailed functional description of the objective, the reader is referred to the patent application PCT/EP00/13148. All lenses of this objective consist of calcium fluoride crystal. The numerical aperture on the image side of the objective is 0.8.

For the exemplary embodiment of FIG. 8, the aperture angles θ and path lengths RL_(L) of the upper outermost aperture ray 713 and of the lower outermost aperture ray 715 were calculated for the individual lenses L801 to L817. The outermost aperture rays 713 and 715 originate from the object point at the coordinates x=0 mm and y=−82.15 mm and have in the image plane angles relative to the optical axis that correspond to the numerical aperture on the image side. The upper and lower outermost aperture rays were calculated because the object field is located outside the optical axis and therefore, like the outermost aperture ray of the exemplary embodiment of FIG. 7, the aperture rays are not symmetrical to the optical axis.

Table 4 lists the data for the upper outermost aperture ray, and Table 5 lists the data for the lower outermost aperture ray. Table 4 and Table 5 list not only the aperture angles θ and optical path lengths RL_(L) for the outermost aperture ray but also the optical path differences for two mutually orthogonal states of linear polarization for different lens orientations; to be precise for (111)-lenses, (100)-lenses and (110)-lenses, the azimuth angle α_(L) of the outermost edge ray within the lenses being 0° and 60° for a (111)-lens, 0° and 45° for a (100)-lens and 0°, 45°, 90° and 135° for a (110)-lens. TABLE 4 Optical path Optical path Optical path Optical path difference difference difference difference Aperture Path length (111)-lens (111)-lens (100)-lens (100)-lens Lens angle θ [°] RL_(L) [mm] α_(L) = 0° [nm] α_(L) = 60° [nm] α_(L) = 0° [nm] α_(L) = 45° [nm] L801 1.4 28.1 0.8 −0.8 0.0 0.0 L802 −10.8 30.7 −5.3 8.0 −2.7 −1.3 L803 −15.6 32.4 −6.8 12.4 −5.7 −2.7 L803 −24.4 31.8 −6.5 17.8 −11.7 −5.2 L802 −19.5 26.6 −5.8 12.4 −6.8 −3.2 L804 6.4 20.1 3.0 −2.4 −0.6 −0.3 L805 10.8 34.4 9.0 −6.0 −3.0 −1.5 L806 0.2 10.0 0.1 −0.1 0.0 0.0 L807 −11.1 22.0 −3.9 5.9 −2.1 −1.0 L808 0.1 18.5 0.0 0.0 0.0 0.0 L809 −0.8 9.0 −0.1 0.2 0.0 0.0 L810 1.1 12.4 0.3 −0.3 0.0 0.0 L811 −16.8 9.4 −2.0 3.8 −1.9 −0.9 L812 −10.4 29.8 −5.0 7.5 −2.4 −1.2 L813 −8.8 34.7 −5.2 7.3 −2.1 −1.0 L814 −9.4 17.5 −2.8 4.0 −1.2 −0.6 L815 −27.4 28.1 −5.3 16.9 −12.2 −5.3 L816 −28.7 40.2 −7.1 24.8 −18.6 −7.9 L817 −30.8 39.0 −6.3 24.7 −19.6 −8.1 Sum −48.9 136.1 −90.9 −40.3 Optical path Optical path Optical path Optical path difference difference difference difference (110)-lens (110)-lens (110)-lens (110)-lens Lens α_(L) = 0° [nm] α_(L) = 45° [nm] α_(L) = 90° [nm] α_(L) = 135° [nm] L801 −18.2 −18.2 −18.2 −18.2 L802 −17.2 −17.2 −17.3 −17.2 L803 −15.3 −15.3 −15.4 −15.3 L803 −8.4 −8.8 −9.0 −8.8 L802 −10.2 −10.3 −10.4 −10.3 L804 −12.4 −12.4 −12.4 −12.4 L805 −19.3 −19.3 −19.3 −19.3 L806 −6.5 −6.5 −6.5 −6.5 L807 −12.2 −12.2 −12.3 −12.2 L808 −12.0 −12.0 −12.0 −12.0 L809 −5.8 −5.8 −5.8 −5.8 L810 −8.0 −8.0 −8.0 −8.0 L811 −4.2 −4.2 −4.2 −4.2 L812 −16.9 −16.9 −16.9 −16.9 L813 −20.5 −20.5 −20.5 −20.5 L814 −10.2 −10.2 −10.2 −10.2 L815 −5.2 −6.4 −6.1 −6.4 L816 −6.2 −8.5 −7.6 −8.5 L817 −3.9 −8.0 −5.7 −8.0 Sum −212.9 −220.9 −218.0 −220.9

TABLE 5 Optical path Optical path Optical path Optical path difference difference difference difference Aperture Path length (111)-lens (111)-lens (100)-lens (100)-lens Lens angle θ [°] RL_(L) [mm] α_(L) = 0° [nm] α_(L) = 60° [nm] α_(L) = 0° [nm) α_(L) = 45° [nm] L801 −11.6 32.1 −5.8 9.0 −3.2 −1.6 L802 19.5 28.3 13.3 −6.1 −7.3 −3.4 L803 24.7 33.8 19.1 −6.9 −12.7 −5.7 L803 17.7 34.3 14.7 −7.4 −7.5 −3.6 L802 12.7 31.6 9.7 −6.0 −3.8 −1.8 L804 −5.2 27.7 −2.7 3.3 −0.6 −0.3 L805 −4.5 34.6 −3.0 3.5 −0.5 −0.3 L806 −8.6 19.5 −2.9 4.0 −1.1 −0.6 L807 −0.5 16.5 −0.2 0.2 0.0 0.0 L808 −8.2 25.6 −3.7 5.0 −1.3 −0.7 L809 −7.5 10.1 −1.3 1.8 −0.4 −0.2 L810 −9.1 13.1 −2.0 2.9 −0.8 −0.4 L811 9.0 9.9 2.1 −1.5 −0.6 −0.3 L812 2.6 30.7 1.8 −1.6 −0.2 −0.1 L813 0.9 34.0 0.6 −0.6 0.0 0.0 L814 1.3 10.4 0.3 −0.3 0.0 0.0 L815 23.5 16.3 8.9 −3.4 −5.7 −2.6 L816 24.6 37.2 21.0 −7.6 −13.9 −6.2 L817 29.4 29.6 18.5 −5.1 −14.1 −5.9 Sum 88.3 −16.8 −73.7 −33.5 Optical path Optical path Optical path Optical path difference difference difference difference (110)-lens (110)-lens (110)-lens (110)-lens Lens α_(L) = 0° [nm] α_(L) = 45° [nm] α_(L) = 90° [nm] α_(L) = 135° [nm] L801 −17.6 −17.6 −17.6 −17.6 L802 −10.9 −10.9 −11.1 −10.9 L803 −8.6 −9.2 −9.3 −9.2 L803 −14.6 −14.6 −14.8 −14.6 L802 −16.7 −16.7 −16.8 −16.7 L804 −17.4 −17.4 −17.4 −17.4 L805 −21.9 −21.9 −21.9 −21.9 L806 −11.6 −11.6 −11.6 −11.6 L807 −10.7 −10.7 −10.7 −10.7 L808 −15.3 −15.3 −15.3 −15.3 L809 −6.1 −6.1 −6.1 −6.1 L810 −7.7 −7.7 −7.7 −7.7 L811 −5.8 −5.8 −5.8 −5.8 L812 −19.8 −19.8 −19.8 −19.8 L813 −22.1 −22.1 −22.1 −22.1 L814 −6.7 −6.7 −6.7 −6.7 L815 −4.7 −4.8 −4.9 −4.8 L816 −9.6 −10.2 −10.3 −10.2 L817 −4.0 −6.2 −5.2 −6.2 Sum −231.9 −235.4 −235.2 −235.4

In accordance with Table 4 and Table 5, the aperture angles θ for the lenses L815 to L817 are larger than 25°. In this exemplary embodiment, again, the lenses L815 to L817 which are closest to the image plane have large aperture angles. Based on the design of the lenses L815 to L817, the maximum aperture angle does not exceed ${\arcsin\left( \frac{NA}{n_{FK}} \right)} = {{\arcsin\left( \frac{0.8}{1.5597} \right)} = {30.9{{^\circ}.}}}$ The maximum aperture angle for the outermost aperture ray is 30.8°, for the lens L817.

The diameter of the diaphragm between the lenses L811 and L812 is 193 mm. The diameter of the lenses L815 to L817 are all smaller than 162 mm. Consequently, the diameters of these lenses, which have large aperture angles, are smaller than 85% of the diaphragm diameter.

As can be concluded from Table 4 and Table 5, it is beneficial for lenses with large aperture angles to orient them in the (100)-direction, because the birefringence values are lower overall. For example, in the lenses L815 to L817, the optical path differences are more than 20% lower.

Based on the exemplary embodiment of FIG. 8, the following discussion is intended to demonstrate how the intrinsic birefringence can to a large extent be compensated by using groups of mutually rotated (100)-lenses in parallel with groups of mutually rotated (111)-lenses.

Firstly, all the calcium fluoride lenses of (111)-orientation are installed without rotating the (111)-lenses with respect to one another. In this case, a maximum optical path difference of 136 nm is obtained for two mutually orthogonal states of linear polarization. By rotating the (111)-lenses, the maximum optical path difference can be reduced to approximately 38 nm. For this purpose, the lenses L801 and L804 are assigned to a group, and the lenses L802 and L803 are assigned to a further group, where the angle of rotation between the lenses in each case is 60°. The lenses L808, L809 and L810 are combined into a group of three, as are the lenses L815, L816 and L817, where the angle of rotation between pairs of these lenses is 40°. The lenses L811, L812, L813 and L814 are combined into a group of four with an angle of mutual rotation of 30°.

If all the calcium fluoride lenses of (100)-orientation are installed without rotating the (100)-lenses with respect to one another, a maximum optical path difference of 90.6 nm is obtained for two mutually orthogonal states of linear polarization. By rotating the (100)-lenses, the maximum optical path difference can be reduced to approximately 40 nm. For this purpose, the lenses L801 and L804 are assigned to a group, and the lenses L802 and L803 are assigned to a further group, where the angle of rotation between the lenses in each case is 45°. The lenses L808, L809 and L810 are combined into a group of three, as are the lenses L815, L816 and L817, where the angle of rotation between pairs of these is 30°. The lenses L811, L812, L813 and L814 are combined into a group of four with an angle of mutual rotation of 22.5°.

A maximum optical path difference of 7 nm is obtained for two mutually orthogonal states of linear polarization if groups of (100)-lenses are combined with groups of (111)-lenses. For this purpose, the lenses L801 and L804 are combined to form a group of (111)-lenses with an angle of rotation of 60° between the lenses. The lenses L802 and L803 are combined to form a group of (100)-lenses with an angle of rotation of 45° between the lenses. The lenses L808, L809 and L810 are combined into a group of three (100)-lenses with an angle of rotation of 30° between respective pairs of these lenses. The lenses L815, L816, L817 are combined to form a group of three (111)-lenses with an angle of rotation of 40° between pairs of these lenses. The lenses L811, L812, L813 and L814 are combined to form a group of four (100)-lenses with an angle of rotation of 22.5°. The lens axes of the lenses L805 and L807 which are not combined to form a group are oriented in the <111> crystallographic direction, while the lens axis of the lens L806 is oriented in the <100> crystallographic direction. The groups can be rotated with respect to one another by an arbitrary angle about the optical axis. These degrees of freedom can be used for the compensation of aberrations from rotational symmetry which can be produced for example by the mounting of the lenses.

The refractive objective 611 is to be used in the following discussion to show how the detrimental influence of birefringence effects can be significantly reduced by covering an optical element with a compensation coating 613. Only the contributions of the two lenses L629 and L630 to the birefringence will be considered here, as these two lenses consist of calcium fluoride and are therefore intrinsically birefringent. In this exemplary embodiment, the two lenses have a (111)-orientation and are rotated by 60° with respect to one another. This leads to an almost rotationally symmetrical distribution of the optical path differences ΔOPL. The maximum optical path difference ΔOPL for an outermost aperture ray is between 13.6 nm and 14.6 nm, depending on the azimuth angle α_(R). The compensation coating 613 described in Table 6 is then applied to the optical surface of the lens L630 that faces toward the image plane O′. The compensation coating 613 consists of 15 individual layers of the materials magnesium fluoride (MgF₂) and lanthanum fluoride (LaF₃). n and k in Table 6 represent the real part and the imaginary part of the refractive index. The layer thicknesses are homogeneous and have no lateral thickness variation. The vapor-depositing angles during the coating process are perpendicular to the optical surface of the lens L630. With the compensation coating, the resultant optical path difference is 1.1 nm and is consequently significantly reduced in comparison with the objective a compensation coating. TABLE 6 Layer Thickness [nm] Material Substrate CaF2 1 103.54 MgF2 2 41.54 LaF3 3 33.35 MgF2 4 30.8 LaF3 5 39.53 MgF2 6 35.34 LaF3 7 32.05 MgF2 8 27.25 LaF3 9 28.57 MgF2 10 26.48 LaF3 11 27.64 MgF2 12 26.17 LaF3 13 27.36 MgF2 14 26.11 LaF3 15 8.66 MgF2 Optical constants n k LaF3 1.760026 0.00118471 MgF2 1.506675 0.00305275

An analogous procedure is also possible if, instead of the two last lenses, the entire objective is considered. Instead of compensating for the birefringence with only one optical element with a compensation coating, it is also possible to cover a plurality of optical elements with compensation coatings.

The procedure can also be used to compensate for birefringence in an entire system, where the causes of this birefringence may be stress-induced birefringence, intrinsic birefringence and birefringence due to the other layers.

Following the final adjustment of a system, the distribution of the optical path differences ΔOPL for one or more bundle of rays in the image plane is determined. The required compensation coating is then calculated by means of a program for coating optimization, and the coating is applied, for example to the system surface closest to the image plane. It is beneficial if the optical element closest to the image plane is interchangeable. This provides the possibility of also correcting birefringence effects that occur only with the actual use of the objective.

In order to compensate for birefringence of crystals in the ultraviolet range, it is possible to arrange crystal elements with different orientations of the crystallographic axes one after the other, as described above. However, if lenses with different crystallographic orientations are arranged one after the other in an optical system, the problem occurs that lenses are often passed through at different angles, so that only a limited degree of compensation may be possible. In the case of optical systems that contain only one crystal lens, this type of compensation is not possible at all.

One possible solution is to design a lens in such a way that it is split into two mutually rotated parts that are joined by wringing. In practice, this concept has the drawback that stresses deform the joining surface and that the two halves have to be laterally positioned at a micrometer level of accuracy.

It is proposed to manufacture blanks from individual plates that are wrung together and rotated relative to the orientation of the crystallographic axes, and then make the blanks into lenses by grinding and polishing. Everything said above about orientations also applies to lenses made in this manner.

In addition to wringing, which is a traditional manufacturing process in the field of optics, any other joining technique that provides intimate contact and introduces a minimal amount of stress can be used and is considered to be encompassed by the invention. The wringing can be facilitated in particular by coatings, for example of quartz glass. It is important to have no refraction or reflection at the joining surface as this would interfere with the intended function.

The orientations are selected on the basis of the criteria described above.

Given as exemplary embodiments are blanks which can be used for example to make the lens L816 for the projection objective of FIG. 8. The lens L816 has a convex aspherical front surface with a vertex curvature radius of 342.13 mm and a concave spherical rear surface with a vertex curvature radius of 449.26 mm. The axial thickness is 37.3 mm. The lens material is calcium fluoride. The lens diameter is 141 mm. The blank from which the lens is to be manufactured requires an overall thickness of at least 45 mm and a diameter of 150 mm. The blank may in this case comprise two (100)-plates of 9.0 mm thickness that are rotated by 45° with respect to one another and two (111)-plates of 13.5 mm thickness that are rotated by 60° with respect to one another, which are joined in an optically seamless manner. The (100)-plates and the (111)-plates should be respectively arranged adjacently.

In a further embodiment, six (100)-plates of 3.0 mm thickness that are rotated with respect to one another by 45° and six (111)-plates of 4.5 thickness that are rotated with respect to one another by 60° are joined in an optically seamless manner, alternating between pairs of (100)-plates and pairs of (111)-plates.

In a further embodiment, four (110)-plates of 9.0 mm thickness that are rotated with respect to one another by 45° and two (100)-plates of 4.5 thickness that are rotated with respect to one another by 45° are joined in an optically seamless manner, with the two (100)-plates following the four (110)-plates.

In a further embodiment, eight (110)-plates of 4.5 mm thickness that are rotated with respect to one another by 45° and four (100)-plates of 2.25 thickness that are rotated with respect to one another by 45° are joined in an optically seamless manner, in a sequence of four (110)-plates and two (100)-plates followed by another four (110)-plates and two (100)-plates.

The construction of a microlithography projection-exposure installation is described in principle on the basis of FIG. 9. The projection-exposure installation 81 has an illuminating device 83 and a projection objective 85. The projection objective 85 comprises a lens arrangement 819 with an aperture diaphragm AP, an optical axis 87 being defined by the lens arrangement 89. Exemplary embodiments of the lens arrangement 89 are given in FIG. 6 and FIG. 7. A mask 89, held in the path of rays by means of a mask holder 811, is arranged between the illuminating device 83 and the projection objective 85. Such masks 89 used in microlithography have a micrometer-to-nanometer structure, a reduced image of which, for example four to five times smaller, is projected by means of the projection objective 85 onto an image plane 813. A light-sensitive substrate 815, or a wafer, positioned by a substrate holder 817 is held in the image plane 813.

The limit of resolution in the projection of fine structures depends on the wavelength λ of the light used for the illumination and on the numerical aperture on the image side of the projection objective 85, the maximum achievable resolution of the projection-exposure installation 81 increasing with decreasing wavelength λ of the illuminating device 83 and with increasing numerical aperture of the projection objective 85 on the image side. With the exemplary embodiments shown in FIG. 6 and FIG. 7, it is possible to achieve resolutions smaller than 150 nm. Therefore, effects such as intrinsic birefringence must also be minimized. The invention has been successful in greatly reducing the detrimental influence of intrinsic birefringence particularly in projection objectives with large numerical apertures on the image side.

The influence of the anisotropy of a compensation coating on the birefringence brought about by the coating is explained on the basis of FIG. 10. In this case, the absolute amount and the direction of the birefringence brought about by the layer are specified by the phase angle ΔPH of the phase splitting, that is by a difference in the wave front between two mutually orthogonal, linear states of polarization. This parameter is suitable also for describing the directional dependence of the birefringence. In FIG. 10, the dependence of the phase angle on the angle of the introduction of the radiation is indicated, corresponding to the aperture angle Θ of the previous representations. The phase splitting brought about by an anisotropic interference layer system (magnesium fluoride lanthanum fluoride multilayer laminate) applied to a planar substrate by being vapor-deposited at a depositing angle of 40° is shown. This is compared with an isotropic layer system.

The curves identified by HOM in the middle indicate the measured values for the isotropic compensation coating 613, which is specified in Table 6 and the layer thicknesses of which are homogeneous and have no lateral thickness variation. As explained, the effective birefringence of the layer, characterized by the phase splitting, for the aperture angle Θ=0° approximates to zero. For higher aperture angles, a slight shift of the phase splitting to negative values occurs on account of the increased path of rays within the layer. The solid line shows the ellipsometric measured values for irradiations in a first plane, which is perpendicular to the coating surface and is designated here as the 0° plane. The broken line indicates the values for the 90° plane perpendicular thereto. It is evident that the amount and direction of the phase splitting are substantially independent of the azimuth angle □. This is consequently an isotropic coating.

In the case of the anisotropic coating (AN) on the other hand, there is a pronounced directional dependence of the phase splitting on the azimuth angle □. The curve AN 0° shows the measured values of directions of irradiation in the first plane, which corresponds to the azimuth angle 0°. In comparison with the homogeneous coating, substantially the same angle variation is evident, but the amount of the birefringence is clearly higher, with a considerable phase splitting (about 10°) even at the angle of irradiation of 0°. Substantially the same values are obtained if radiation is introduced in the same first plane from the direction lying opposite with respect to the optical axis. This corresponds to rotation of the sample with respect to the measuring device by 180° (curve AN 180°).

If, on the other hand, radiation is introduced in a plane perpendicular to the first plane (curve AN 90°), the azimuth angle therefore being changed by 90°, although there is the same phase splitting in terms of the magnitude, it is with a negative phase angle. This demonstrates that, with the aid of an anisotropic coating, the direction of the phase splitting can be controlled by producing an anisotropic coating with a specific preferred direction, which is then to be aligned in a prescribed direction in relation to the electrical field vectors of the incoming radiation.

It is explained on the basis of FIG. 11 that, in the case of anisotropic coatings, the extent of the phase splitting, i.e. the intensity of the birefringence, can also be selectively controlled. Measured values for the phase splitting in dependence on the aperture angle Θ for different coatings are shown. The lozenge symbols in this case correspond to an MgF2/LaF3 multilayer laminate with 8 individual layers, vapor-deposited at 150° C. The square symbols correspond to a coating produced at the same temperature with 6 individual layers of these materials. A comparison of the birefringent effect of these two layer systems shows that the absolute amount of the achievable phase splitting increases as the number of layers increases.

The triangle symbols correspond to a coating with 8 layers, coated at 250°, that is much higher temperatures. In comparison with the corresponding 8-ply layer at 150°, much lower phase splittings are obtained.

These tendencies (increase of the birefringent effect with increasing number of layers, lessening of the birefringent effect with increasing production temperature) also become clear from the comparison with the other layer systems (six-layer system at room temperature, two-layer system at 220° and 150°).

On the basis of the dependences represented by way of example, it is possible with the aid of anisotropic coatings to create polarization-effective optical components which have a prescribable locally varying birefringence with respect to amount and direction over the coated surface area. The production of a rotationally symmetrical anisotropic coating of a lens in a vapor-depositing installation with a planetary system is explained on the basis of FIG. 12. The planetary system has a main carrier which can be rotated about a main axis of rotation 500 (not shown) and attached to the circumference of which are a number of substrate carriers 502, which are rotatable about respective substrate carrier axes 501. Each substrate carrier carries a substrate 503, which in the example is designed as a biconvex lens. Arranged in the region of the main axis of rotation is a material source 504 for vapor-depositing material, in order for example to evaporate magnesium fluoride and lanthanum fluoride alternately with the aid of an electron beam and vapor-deposit them onto the coating surface 505 of the substrate facing the material source. The coating material symbolized by broken lines meets the respective coating locations at a vapor-depositing angle (coating angle) 506, which is determined by the geometry of the installation and the curvature of the coating surface.

In order to produce an anisotropic coating 510 in this installation, arranged between the material source 504 and the substrate is a set of shading diaphragms 511, with each of which the part of the coating surface facing the material source can be shaded completely from the stream of material, so that only the part of the coating surface facing away from the material source is coated at a large vapor-depositing angle.

The oblique vapor-depositing has the effect that the layer material grows up in a columnar structure, symbolized by oblique lines. The angle of inclination is in this case determined by the principal vapor-depositing direction. The degree of this morphology can be influenced by the coating temperature, the anisotropy being pronounced to a greater degree at lower coating temperatures, for example between room temperature and about 90° C., and at higher coating temperatures, for example of over 120° C., 150° C. or 200° C.

It is evident that in this way it is possible to produce an anisotropic coating which is rotationally symmetrical to the axis of rotation 501 and in which the preferred direction (direction of inclination of the coating material columns) points substantially in the radial direction. On account of the curvature of the lens surface, a variation of the vapor-depositing angle is also obtained from the center to the edge, this angle increasing in the example from the inside to the outside, so that the anisotropy of the coating is greater at the edge than in the center. In the case of a concave surface, the situation would be reversed. It is also evident that, by setting the difference in height between the material source 504 and the coating surface, the concentration of the distribution of the angles of incidence can be set, smaller vertical distances leading to larger vapor-depositing angles.

It is explained on the basis of FIG. 13 how, by suitably designing shading diaphragms for each coating location, a coating time that is dependent on the radial position of the coating location can be prescribed according to a prescribable radial time profile and a desired angular spectrum of the directions of growth of the columnar structures. The shading effect of the diaphragms is in this case symbolically represented by diaphragms 511′, 511″, 511′″, which periodically shade a substrate 503′, 503″, 503′″ rotating behind them during its rotation about the axis of rotation 501′, 501″, 501′″ with respect to the stream of material 520, represented by arrow symbols, of the material source.

FIG. 13 (a) shows a shading with a shading angle which is constant over the entire radius and is measured in the circumferential direction of the rotation. This is achieved by a V-shaped “window” in the stream of material. This shading has the effect that the coating time, i.e. the time for which the rotating coating location lies within the stream of material 520, is substantially the same for all radial positions. In the case of planar substrate surfaces, this would lead to a structure which is largely uniform in the radial direction. If the shading window were expanded outward, for example according to the broken lines 515, this would produce a radial time profile with which radially inner regions are shaded for longer than radially outer regions. This makes it possible, if appropriate, to achieve compensation for geometrically caused decreases in layer thickness toward the edge. However, it is also possible to produce rotationally symmetrical layers, the layer thickness of which increases continuously from the center to the edge. The narrower the range of the vapor-depositing directions allowed by the shading diaphragm, the narrower or smaller the angular spectrum of the directions of growth of the columnar structures. A narrower angular spectrum in this case generally corresponds to a more pronounced anisotropy.

The rectangular window in FIG. 13 (b) has the effect that the radially inner regions are vapor-deposited over longer time intervals and from more varied directions than radially outer regions. This makes it possible, if appropriate, to produce layers of which the layer thickness decreases more from the center to the edge than a thickness decrease caused by the surface curvature alone. On the other hand, the anisotropy will be more pronounced at the edge than in the center. In the case of concave coating surfaces, it is possible in this way, if appropriate, to achieve a homogeneous layer thickness between the center and the edge.

The geometry of the diaphragms 511′″ in FIG. 13 (c) has the effect that a central region 525 of the substrate 503′″ remains constantly exposed to the stream of material and is consequently coated substantially isotropically. In the remaining region, a radial variation is obtained, with the anisotropy and/or layer thickness changing in the radial direction.

The alignment of the principal axis of the coating can be set by suitable choice of the orientation of the stream of vapor-deposited material with respect to the diaphragms. If, for example, in the case of the arrangement according to FIG. 13 (a) vapor-depositing is carried out in a direction offset by 90° (broken arrow), columnar structures which are aligned substantially tangentially result.

In order to hinder or avoid penetration of impurities, such as hydrocarbons, water vapor or the like, into the porous layer structure, a largely pore-free protective layer may be applied, for example as an outermost layer of the coating, serving as a diffusion barrier. This may be formed, by a suitable layer thickness, such that it is optically largely neutral, for example as a half-wave layer.

A variant of the method which makes it possible to produce polarization-optically effective components (for example retarders) with a virtually freely selectable local birefringence distribution is explained on the basis of FIG. 15. For this purpose, firstly a substrate, for example a planar-parallel plate 550, is covered with a polarization-optically effective coating 551, the birefringent effect of which may be substantially homogeneous or anisotropic. The coating has a non-equilibrium layer structure, which allows the morphology of the layer, and consequently its birefringent properties, to be changed in the exposed region by selective local introduction of energy. This may be, as in the example, an anisotropic coating 551 produced by oblique vapor-deposition. After completion of the layer 551, the layer is locally exposed in accordance with a prescribed distribution to an energy which is suitable for changing the morphology, and consequently the birefringent properties of the layer material. In the example, for this purpose a high-energy radiation 552, for example an ion beam or a suitably expanded laser beam, is introduced through the aperture 553 of a mask 554 onto the coating. Induced as a result in the region 555 of the coating prescribed by the shape of the mask aperture 553 is a diffusion-supported change of the morphology, in which for example the columnar growth structures of an anisotropic layer coagulate and form a layer of greater density and less anisotropy. A number of masks with different forms of aperture may also be used one after the other, in order to produce more complex local birefringent distributions. Maskless methods are also possible, for example by “writing” a desired birefringent distributions with the aid of a focused high-energy beam, for example a laser beam. In this way, virtually any desired local modulations of the phase splitting brought about by a coating can be produced. The regions of uniform phase splitting may be of a very small scale, for example a few millimeters.

With the method, polarization masks (i.e. polarizers or retardation elements with a locally dependent effect) can be produced for various wavelength ranges, in particular by using anisotropic starting layers. Starting coatings for the visible wavelength range can be produced for example by “cold vapor-depositing”, it then being possible for the morphology change to be produced by irradiation with high-energy laser light, for example from the UV range. The starting layers must be stable with respect to the working wavelength even for applications in the UV range, in particular in the case of a wavelength below about 260 nm. Here it is expedient to produce the starting coating at higher coating temperatures, for example between 100° and 152°, whereby thermally more stable coatings are created. The transformation of the morphology must be carried out with correspondingly greater introductions of energy, for example with an infrared laser, by ion beams, electron beams or by suitable heating dies.

The methods described are also suitable for producing polarizing phase-shift masks as polarization-optically effective components, in particular using at least one anisotropic coating. In this case, for example, regions in the direct vicinity of structures to be replicated, for example a line, can be covered with a spatially delimited anisotropic coating in order to produce a favorable retarder effect. The construction and functional principle of polarizing phase shift masks (polarized phase shift masks, P:PSM) are described for example in the article: “Polarized Phase Shift Mask: Concept, Design and Potential Advantages to Photolithography Process and Physical Design” by R. Wang, W. Grobmann, A. Reich and M. Thompson, Proc. SPIE Vol. 4562, pages 406 et seq, the disclosure of which is made the content of this description by reference. With the aid of such masks, it is possible if appropriate to mitigate or eliminate the problem of “phase conflict”, so that a single exposure may possibly be adequate for replication of adequate quality. TABLE 1 M1587a REFRACTIVE FREE INDEX ½ AT LENS RADII THICKNESSES GLASSES 157.629 nm DIAMETER 0 0.000000000 27.171475840 N2 1.00031429 46.200 0.000000000 0.602670797 N2 1.00031429 52.673 L601 900.198243311AS 15.151284556 CaF2 1.55929035 53.454 −235.121108435 9.531971079 N2 1.00031429 54.049 L602 −167.185917779 8.294716452 CaF2 1.55929035 54.178 −132.673519510 14.020355779 N2 1.00031429 54.901 L603 −333.194588652 9.893809820 CaF2 1.55929035 53.988 −155.450516203 15.930502944 N2 1.00031429 54.132 L604 −73.572316296 7.641977580 CaF2 1.55929035 53.748 −68.248613899AS 2.881720302 N2 1.00031429 55.167 L605 −86.993585564AS 5.094651720 CaF2 1.55929035 52.580 −238.150965327 5.379130780 N2 1.00031429 53.729 L606 −165.613920870 5.094651720 CaF2 1.55929035 53.730 153.417884485 34.150169591 N2 1.00031429 56.762 L607 −92.061009990 5.094651720 CaF2 1.55929035 58.081 8491.086261873AS 19.673523795 N2 1.00031429 74.689 L608 −407.131300451 30.380807138 CaF2 1.55929035 87.291 −140.620317156 0.761662684 N2 1.00031429 91.858 L609 −4831.804853654AS 50.269660218 CaF2 1.55929035 117.436 −192.197373609 1.688916911 N2 1.00031429 121.408 L610 −367.718684892 21.227715500 CaF2 1.55929035 127.704 −233.628547894 2.224071019 N2 1.00031429 129.305 L611 709.585855080 28.736922725 CaF2 1.55929035 137.016 1238.859445357 9.120684720 N2 1.00031429 137.428 L612 1205.457051945 49.281218258 CaF2 1.55929035 138.288 −285.321880705 1.625271224 N2 1.00031429 138.379 L613 137.549591710 56.718543740 CaF2 1.55929035 108.652 −4380.301012978AS 0.623523902 N2 1.00031429 106.138 L614 2663.880214408 6.792868960 CaF2 1.55929035 103.602 149.184979730 15.779049257 N2 1.00031429 84.589 L615 281.093108064 6.792868960 CaF2 1.55929035 83.373 184.030288413 32.341552355 N2 1.00031429 77.968 L616 −222.157416308 5.094651720 CaF2 1.55929035 77.463 101.254238115AS 56.792834221 N2 1.00031429 71.826 L617 −106.980638018 5.094651720 CaF2 1.55929035 72.237 1612.305471130 20.581065398 K2 1.00031429 89.760 L618 −415.596135628 26.398111993 CaF2 1.55929035 96.803 −204.680044631 0.713343960 N2 1.00031429 103.409 L619 −646.696622394 25.867340760 CaF2 1.55929035 116.636 −231.917626896 0.766268682 N2 1.00031429 118.569 L620 −790.657607677 23.400482872 CaF2 1.55929035 128.806 −294.872053725 0.721402031 N2 1.00031429 130.074 L621 786.625567756 40.932308205 CaF2 1.55929035 141.705 −431.247283013 12.736629300 N2 1.00031429 142.089 0.000000000 −8.491086200 N2 1.00031429 134.586 L622 295.022653593AS 20.185109438 CaF2 1.55929035 139.341 449.912291916 0.619840486 K2 1.00031429 137.916 L623 358.934076212 48.662890509 CaF2 1.55929035 136.936 −622.662988878 30.955714157 N2 1.00031429 135.288 L624 −224.404889753 12.736629300 CaF2 1.55929035 134.760 −251.154571510AS 16.079850229 N2 1.00031429 134.853 L625 −193.582989843AS 16.510083506 CaF2 1.55929035 134.101 −198.077570749 0.880353872 N2 1.00031429 136.109 L626 206.241795157 19.927993542 CaF2 1.55929035 101.240 338.140581666 0.925956949 N2 1.00031429 97.594 L627 11.017549581 24.580089962 CaF2 1.55929035 85.023 169.576109839 0.777849447 N2 1.00031429 81.164 L628 117.982165264 31.161065630 CaF2 1.55929035 75.464 921.219058213AS 6.934980174 N2 1.00031429 69.501 L629 0.000000000 22.260797322 CaF2 1.55929035 63.637 0.000000000 4.245543100 N2 1.00031429 48.606 L630 0.000000000 21.227715500 CaF2 1.55929035 41.032 0.000000000 8.491086200 N2 1.00031429 26.698 0.000000000 0.000000000 1.00000000 11.550 ASPHERICAL CONSTANTS Asphere of lens L601 K 0.0000      C1 1.28594437e−007 C2 8.50731836e−013 C3 1.16375620e−016 C4 2.28674275e−019 C5 −1.23202729e−022  C6 3.32056239e−026 C7 −4.28323389e−030  C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L604 K −1.3312      C1 −4.03355456e−007  C2 2.25776586e−011 C3 −2.19259878e−014  C4 4.32573397e−018 C5 −7.92477159e−022  C6 7.57618874e−026 C7 −7.14962797e−030  C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L605 K −1.1417      C1 1.33637337e−007 C2 1.56787758e−011 C3 −1.64362484e−014  C4 3.59793786e−018 C5 −5.11312568e−022  C6 1.70636633e−026 C7 1.82384731e−030 C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L607 K 0.0000      C1 1.34745120e−007 C2 −2.19807543e−011  C3 1.20275881e−015 C4 4.39597377e−020 C5 −2.37132819e−023  C6 2.87510939e−027 C7 −1.42065162e−031  C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L609 K 0.0000      C1 6.85760526e−009 C2 −4.84524868e−013  C3 −6.28751350e−018  C4 −3.72607209e−022  C5 3.25276841e−026 C6 −4.05509974e−033  C7 −3.98843079e−035  C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L613 K 0.0000      C1 2.24737416e−008 C2 −4.45043770e−013  C3 −4.10272049e−017  C4 4.31632628e−021 C5 −3.27538237e−025  C6 1.44053025e−029 C7 −2.76858490e−034  C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L616 K 0.0000      C1 −2.83553693e−008  C2 −1.12122261e−011  C3 −2.05192812e−016  C4 −1.55525080e−020  C5 −4.77093112e−024  C6 8.39331135e−028 C7 −8.97313681e−032  C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L622 K 0.0421      C1 7.07310826e−010 C2 −2.00157185e−014  C3 −9.33825109e−020  C4 1.27125854e−024 C5 1.94008709e−027 C6 −6.11989858e−032  C7 2.92367322e−036 C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L624 K 0.0000      C1 3.02835805e−010 C2 −2.40484062e−014  C3 −3.22339189e−019  C4 1.64516979e−022 C5 −8.51268614e−027  C6 2.09276792e−031 C7 −4.74605669e−036  C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L625 K 0.0000      C1 −3.99248993e−010  C2 5.79276562e−014 C3 3.53241478e−018 C4 −4.57872308e−023  C5 −6.29695208e−027  C6 1.57844931e−031 C7 −2.19266130e−036  C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L628 K 0.0000      C1 4.40737732e−008 C2 1.52385268e−012 C3 −5.44510329e−016  C4 6.32549789e−020 C5 −4.58358203e−024  C6 1.92230388e−028 C7 −3.11311258e−033  C8 0.00000000e+000 C9 0.00000000e+000 Wavelength and refractive index are stated relative to vacuum.

TABLE 3 L61 REFRACTIVE FREE INDEX ½ AT LENS RADII THICKNESSES GLASSES 157.13 nm DIAMETER 0 0.000000000 34.000000000 1.00000000 82.150 0.000000000 0.100000000 1.00000000 87.654 L801 276.724757380 40.000000000 CaF2 1.55970990 90.112 1413.944109416AS 95.000000000 1.00000000 89.442 SP1 0.000000000 11.000000000 1.00000000 90.034 0.000000000 433.237005445 1.00000000 90.104 L802 −195.924336384  17.295305525 CaF2 1.55970990 92.746 −467.658808527  40.841112468 1.00000000 98.732 L803 −241.385736441  15.977235467 CaF2 1.55970990 105.512 −857.211727400AS 21.649331094 1.00000000 118.786 SP2 0.000000000 0.000010000 1.00000000 139.325 253.074839896 21.649331094 1.00000000 119.350 L803′ 857.211727400AS 15.977235467 CaF2 1.55970990 118.986 241.385736441 40.841112468 1.00000000 108.546 L802′ 467.658808527 17.295305525 CaF2 1.55970990 102.615 195.924336384 419.981357165 1.00000000 95.689 SP3 0.000000000 6.255658280 1.00000000 76.370 0.000000000 42.609155219 1.00000000 76.064 Z1 0.000000000 67.449547115 1.00000000 73.981 L804 432.544479547 37.784311058 CaF2 1.55970990 90.274 −522.188532471  113.756133662 1.00000000 92.507 L805 −263.167605725  33.768525968 CaF2 1.55970990 100.053 −291.940616829AS 14.536591424 1.00000000 106.516 L806 589.642961222AS 20.449887046 CaF2 1.55970990 110.482 −5539.698828792  443.944079795 1.00000000 110.523 L807 221.780582003 9.000000000 CaF2 1.55970990 108.311 153.071443064 22.790060084 1.00000000 104.062 L808 309.446967518 38.542735318 CaF2 1.55970990 104.062 −2660.227900099  0.100022286 1.00000000 104.098 L809 23655.354584194  12.899131182 CaF2 1.55970990 104.054 −1473.189213176  9.318886362 1.00000000 103.931 L810 −652.136459374  16.359499814 CaF2 1.55970990 103.644 −446.489459129  0.100000000 1.00000000 103.877 L811 174.593507050 25.900313780 CaF2 1.55970990 99.267 392.239615259AS 14.064505431 1.00000000 96.610 0.000000000 2.045119392 1.00000000 96.552 L812 7497.306838492  16.759051656 CaF2 1.55970990 96.383 318.210831711 8.891640764 1.00000000 94.998 L813 428.724465129 41.295806263 CaF2 1.55970990 95.548 3290.097860119AS 7.377912006 1.00000000 95.040 L814 721.012739719 33.927118706 CaF2 1.55970990 95.443 −272.650872353  6.871397517 1.00000000 95.207 L815 131.257556743 38.826450065 CaF2 1.55970990 81.345 632.112566477AS 4.409527396 1.00000000 74.847 L816 342.127616157AS 37.346293509 CaF2 1.55970990 70.394 449.261078744 4.859754445 1.00000000 54.895 L817 144.034814702 34.792179308 CaF2 1.55970990 48.040 −751.263321098AS 11.999872684 1.00000000 33.475 0′ −0.000000000 0.000127776 1.00000000 16.430 ASPHERICAL CONSTANTS Asphere of lens L801 K 0.0000 C1 4.90231706e−009 C2 3.08634889e−014 C3 −9.53005325e−019  C4 −6.06316417e−024  C5 6.11462814e−028 C6 −8.64346302e−032  C7 0.00000000e+000 C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L803 K 0.0000      C1 −5.33460884e−009  C2 9.73867225e−014 C3 −3.28422058e−018  C4 1.50550421e−022 C5 0.00000000e+000 C6 0.00000000e+000 C7 0.00000000e+000 C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L803′ K 0.0000      C1 5.33460884e−009 C2 −9.73867225e−014  C3 3.28422058e−018 C4 −1.50550421e−022  C5 0.00000000e+000 C6 0.00000000e+000 C7 0.00000000e+000 C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L805 K 0.0000      C1 2.42569449e−009 C2 3.96137865e−014 C3 −2.47855149e−018  C4 7.95092779e−023 C5 0.00000000e+000 C6 0.00000000e+000 C7 0.00000000e+000 C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L806 K 0.0000      C1 −6.74111232e−009  C2 −2.57289693e−014  C3 −2.81309020e−018  C4 6.70057831e−023 C5 5.06272344e−028 C6 −4.81282974e−032  C7 0.00000000e+000 C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L811 K 0.0000      C1 2.28889624e−008 C2 −1.88390559e−014  C3 2.86010656e−017 C4 −3.18575336e−021  C5 1.45886017e−025 C6 −1.08492931e−029  C7 0.00000000e+000 C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L813 K 0.0000      C1 3.40212872e−008 C2 −1.08008877e−012  C3 4.33814531e−017 C4 −7.40125614e−021  C5 5.66856812e−025 C6 0.00000000e+000 C7 0.00000000e+000 C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L815 K 0.0000      C1 −3.15395039e−008  C2 4.30010133e−012 C3 3.11663337e−016 C4 −3.64089769e−020  C5 1.06073268e−024 C6 0.00000000e+000 C7 0.00000000e+000 C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L816 K 0.0000      C1 −2.16574623e−008  C2 −6.67182801e−013  C3 4.46519932e−016 C4 −3.71571535e−020  C5 0.00000000e+000 C6 0.00000000e+000 C7 0.00000000e+000 C8 0.00000000e+000 C9 0.00000000e+000 Asphere of lens L817 K 0.0000      C1 2.15121397e−008 C2 −1.65301726e−011  C3 −5.03883747e−015  C4 1.03441815e−017 C5 −6.29122773e−021  C6 1.44097714e−024 C7 0.00000000e+000 C8 0.00000000e+000 C9 0.00000000e+000 

1-30. (canceled)
 31. An objective comprising: a plurality of optical elements, each of the plurality of optical elements having optical surfaces, wherein during use an image point in an image plane is impinged by a bundle of rays with rays which respectively have an optical path difference ΔOPL for two mutually orthogonal states of linear polarization, at least one optical surface is covered with a compensation coating, the compensation coating is designed in such a way that the optical path differences ΔOPL of the bundle of rays have significantly reduced values in comparison with an objective without a compensation coating, and the compensation coating comprises multiple layers.
 32. The objective as claimed in claim 31, wherein the optical element with the compensation coating has an element axis, and the compensation coating has an effective birefringence distribution, the effective birefringence values of which depend on azimuth angles α_(F) with respect to a reference direction perpendicular to the element axis and on aperture angles θ_(F) with respect to the element axis.
 33. The objective as claimed in claim 32, wherein the effective birefringence distribution of the compensation coating for the aperture angle θ_(F)=0° is approximately zero.
 34. The objective as claimed in claim 32, wherein the effective birefringence distribution of the compensation coating primarily depends on the aperture angle θ_(F.)
 35. The objective as claimed in claim 32, wherein the optical element with the compensation coating is interchangeable.
 36. The objective as claimed in claim 31, wherein at least two optical elements comprise fluoride crystal lenses are lenses or lens parts each of the lenses or the lens parts has a lens axis, and the lenses or the lens parts are arranged such that they are rotated with respect to one another about the lens axes in such a way that the distribution of the optical path differences ΔOPL (α_(R), θ_(R)) of the bundle of rays as a function of the azimuth angle α_(R) and of the aperture angle θ_(R) has significantly reduced values in comparison with lenses or lens parts of which the lens axes point in the same principal crystallographic direction and which are not arranged such that they are rotated with respect to one another about the lens axes.
 37. The objective as claimed in claim 36, wherein the optical path differences ΔOPL as a function of the azimuth angle α_(R) for a given aperture angle θ_(φ) vary less than 30%.
 38. The objective as claimed in claim 36, wherein the lenses or lens parts respectively have a birefringence distribution Δn (α_(L),θ_(L)), the birefringence values Δn of which depend on azimuth angles α_(L) with respect to a reference direction perpendicular to the lens axis and on aperture angles θ_(R) with respect to the lens axis, the birefringence distribution Δn (α_(L),θ_(L)) having a k-fold azimuthal symmetry, angles of rotation γ being defined between the reference directions of the individual lenses or lens parts, a number of n lenses or n lens parts forming a group within which the lens axes point in the same principal crystallographic direction or a principal crystallographic direction equivalent thereto and within which the birefringence distributions Δn (α_(L),θ_(L)) with respect to the reference directions have the same azimuthal profile, the following applying for the angle of rotation .gamma. between respective pairs of lenses or lens parts of a group: ${\gamma = {\frac{360{^\circ}}{k\quad n} + {{m\quad\frac{360{^\circ}}{k}} \pm {10{^\circ}}}}},$ m being an integral number.
 39. The objective as claimed in claim 36, wherein the optical element with the compensation coating is one of the of fluoride crystal lenses or lens parts, and the element axis is the lens axis of the one of the fluoride crystal lenses or lens parts.
 40. The objective as claimed in claim 31, wherein a plurality of optical element are covered with compensation coatings.
 41. The objective as claimed in claim 31, wherein the objective has a numerical aperture on the image side that is greater than 0.7.
 42. The objective as claimed in claim 31, wherein the objective is designed for wavelengths less than 200 nm.
 43. The objective as claimed in claim 31, wherein the objective is designed for wavelengths less than 160 nm.
 44. The objective as claimed in claim 31, wherein the objective is a refractive objective.
 45. The objective as claimed in claim 31, wherein the objective is a catadioptric objective with lenses and at least one mirror.
 46. The objective as claimed in claim 31, wherein all the lenses are of calcium fluoride.
 47. A microlithography projection-exposure installation, comprising: an illuminating system, an objective as claimed in claim 31, which during use can project an image of a structure-bearing mask onto a light-sensitive substrate.
 48. A method for producing semiconductor components that comprises using a microlithography projection-exposure installation as claimed in claim
 47. 49-81. (canceled)
 82. The objective as claimed in claim 31, wherein at least one of the multiple layers comprises a fluoride.
 83. The objective as claimed in claim 82, wherein the fluoride is selected from the group consisting of magnesium fluoride and lanthanum fluoride.
 84. The objective as claimed in claim 31, wherein at least two of the multiple layers comprise different materials.
 85. The objective as claimed in claim 31, wherein the multiple layers comprise alternating layers of material.
 86. An optical system, comprising: a plurality of optical elements; and a coating comprising multiple layers, wherein: the coating is supported by one of the plurality of optical elements; during use of the optical system the coating at least partially compensates for intrinsic birefringence of at least one of the plurality of optical elements; and the optical system is a microlithography projection objective.
 87. The optical system of claim 86, wherein the coating comprises a fluoride.
 88. The optical system of claim 87, wherein the fluoride is selected from the group consisting of magnesium fluoride and lanthanum fluoride.
 89. The optical system of claim 86, wherein at least two of the multiple layers comprise different materials.
 90. The optical system of claim 86, wherein the one of the plurality of optical elements is a lens.
 91. The optical system of claim 90, wherein the lens is a calcium fluoride lens.
 92. The optical system of claim 86, wherein the material from which the one of the plurality of optical elements is formed comprises calcium fluoride.
 93. The optical system of claim 86, wherein the multiple layers comprise alternating layers of material.
 94. An optical system, comprising: a plurality of optical elements; and a coating, wherein: the coating is supported by one of the plurality of optical elements; the coating comprises at least one material different from the material from which the one of the plurality of optical elements is formed; during use of the optical system the coating at least partially compensates for intrinsic birefringence of at least one of the plurality of optical elements; and the optical system is a microlithography projection objective.
 95. The optical system of claim 94, wherein the coating comprises a fluoride.
 96. The optical system of claim 95, wherein the fluoride is selected from the group consisting of magnesium fluoride and lanthanum fluoride.
 97. The optical system of claim 94, wherein the coating comprises multiple layers.
 98. The optical system of claim 94, wherein the one of the plurality of optical elements is a lens.
 99. The optical system of claim 98, wherein the lens is a calcium fluoride lens.
 100. The optical system of claim 94, wherein the material from which the one of the plurality of optical elements is formed comprises calcium fluoride.
 101. The optical system of claim 86, wherein the multiple layers comprise alternating layers of material. 